clang, cc-wrapper: Move `--gcc-toolchain` logic into CC wrapper

Take 2, after #94582 had to be reverted.

This reverts commit ac03cfa3c5.

pkgs/build-support/cc-wrapper/default.nix

@@ -290,6 +290,17 @@ stdenv.mkDerivation {
       echo "-L${gccForLibs.lib}/${targetPlatform.config}/lib" >> $out/nix-support/cc-ldflags
     ''
 
+    # TODO We would like to connect this to `useGccForLibs`, but we cannot yet
+    # because `libcxxStdenv` on linux still needs this. Maybe someday we'll
+    # always set `useLLVM` on Darwin, and maybe also break down `useLLVM` into
+    # fine-grained use flags (libgcc vs compiler-rt, ld.lld vs legacy, libc++
+    # vs libstdc++, etc.) since Darwin isn't `useLLVM` on all counts. (See
+    # https://clang.llvm.org/docs/Toolchain.html for all the axes one might
+    # break `useLLVM` into.)
+    + optionalString (isClang && gccForLibs != null && targetPlatform.isLinux && !(stdenv.targetPlatform.useLLVM or false)) ''
+      echo "--gcc-toolchain=${gccForLibs}" >> $out/nix-support/cc-cflags
+    ''
+
     ##
     ## General libc support
     ##

pkgs/development/compilers/llvm/10/default.nix

@@ -26,8 +26,6 @@ let
       ln -s "${targetLlvmLibraries.compiler-rt.out}/lib" "$rsrc/lib"
       ln -s "${targetLlvmLibraries.compiler-rt.out}/share" "$rsrc/share"
       echo "-resource-dir=$rsrc" >> $out/nix-support/cc-cflags
-    '' + lib.optionalString (stdenv.targetPlatform.isLinux && !(stdenv.targetPlatform.useLLVM or false)) ''
-      echo "--gcc-toolchain=${gccForLibs}" >> $out/nix-support/cc-cflags
     '';
   in {
 

pkgs/development/compilers/llvm/11/default.nix

@@ -28,8 +28,6 @@ let
       ln -s "${targetLlvmLibraries.compiler-rt.out}/lib" "$rsrc/lib"
       ln -s "${targetLlvmLibraries.compiler-rt.out}/share" "$rsrc/share"
       echo "-resource-dir=$rsrc" >> $out/nix-support/cc-cflags
-    '' + lib.optionalString (stdenv.targetPlatform.isLinux && !(stdenv.targetPlatform.useLLVM or false)) ''
-      echo "--gcc-toolchain=${gccForLibs}" >> $out/nix-support/cc-cflags
     '';
   in {
 

pkgs/development/compilers/llvm/5/default.nix

@@ -25,8 +25,6 @@ let
       ln -s "${cc}/lib/clang/${release_version}/include" "$rsrc"
       ln -s "${targetLlvmLibraries.compiler-rt.out}/lib" "$rsrc/lib"
       echo "-resource-dir=$rsrc" >> $out/nix-support/cc-cflags
-    '' + lib.optionalString (stdenv.targetPlatform.isLinux && !(stdenv.targetPlatform.useLLVM or false)) ''
-      echo "--gcc-toolchain=${gccForLibs}" >> $out/nix-support/cc-cflags
     '';
   in {
 

pkgs/development/compilers/llvm/6/default.nix

@@ -25,8 +25,6 @@ let
       ln -s "${cc}/lib/clang/${release_version}/include" "$rsrc"
       ln -s "${targetLlvmLibraries.compiler-rt.out}/lib" "$rsrc/lib"
       echo "-resource-dir=$rsrc" >> $out/nix-support/cc-cflags
-    '' + lib.optionalString (stdenv.targetPlatform.isLinux && !(stdenv.targetPlatform.useLLVM or false)) ''
-      echo "--gcc-toolchain=${gccForLibs}" >> $out/nix-support/cc-cflags
     '';
   in {
 

pkgs/development/compilers/llvm/7/default.nix

@@ -25,8 +25,6 @@ let
       ln -s "${cc}/lib/clang/${release_version}/include" "$rsrc"
       ln -s "${targetLlvmLibraries.compiler-rt.out}/lib" "$rsrc/lib"
       echo "-resource-dir=$rsrc" >> $out/nix-support/cc-cflags
-    '' + lib.optionalString (stdenv.targetPlatform.isLinux && !(stdenv.targetPlatform.useLLVM or false)) ''
-      echo "--gcc-toolchain=${gccForLibs}" >> $out/nix-support/cc-cflags
     '';
   in {
 

pkgs/development/compilers/llvm/8/default.nix

@@ -25,8 +25,6 @@ let
       ln -s "${cc}/lib/clang/${release_version}/include" "$rsrc"
       ln -s "${targetLlvmLibraries.compiler-rt.out}/lib" "$rsrc/lib"
       echo "-resource-dir=$rsrc" >> $out/nix-support/cc-cflags
-    '' + lib.optionalString (stdenv.targetPlatform.isLinux && !(stdenv.targetPlatform.useLLVM or false)) ''
-      echo "--gcc-toolchain=${gccForLibs}" >> $out/nix-support/cc-cflags
     '';
   in {
 

pkgs/development/compilers/llvm/9/default.nix

@@ -25,8 +25,6 @@ let
       ln -s "${cc}/lib/clang/${release_version}/include" "$rsrc"
       ln -s "${targetLlvmLibraries.compiler-rt.out}/lib" "$rsrc/lib"
       echo "-resource-dir=$rsrc" >> $out/nix-support/cc-cflags
-    '' + lib.optionalString (stdenv.targetPlatform.isLinux && !(stdenv.targetPlatform.useLLVM or false)) ''
-      echo "--gcc-toolchain=${gccForLibs}" >> $out/nix-support/cc-cflags
     '';
   in {
 

pkgs/development/compilers/llvm/multi.nix

@@ -13,7 +13,12 @@ let
     chmod u+rw -R $out/lib
     cp -r ${basegcc.libc}/lib/* $(ls -d $out/lib/gcc/*/*)
   '';
-  gcc_multi_sysroot = runCommand "gcc-multi-sysroot" {} ''
+  gcc_multi_sysroot = runCommand "gcc-multi-sysroot" {
+    passthru = {
+      inherit (gcc64) version;
+      lib = gcc_multi_sysroot;
+    };
+  } ''
     mkdir -p $out/lib/gcc
 
     ln -s ${combine gcc64}/lib/gcc/* $out/lib/gcc/
@@ -32,17 +37,16 @@ let
   '';
 
   clangMulti = clang.override {
-    # Only used for providing expected structure re:dynamic linkers, AFAIK
-    # Most of the magic is done by setting the --gcc-toolchain option below
+    # Only used for providing expected structure re:dynamic linkers, AFAIK Most
+    # of the magic is done by setting the --gcc-toolchain option via
+    # `gccForLibs`.
     libc = gcc_multi_sysroot;
 
     bintools = clang.bintools.override {
       libc = gcc_multi_sysroot;
     };
 
-    extraBuildCommands = ''
-      sed -e '$a --gcc-toolchain=${gcc_multi_sysroot}' -i $out/nix-support/libc-cflags
-    '';
+    gccForLibs = gcc_multi_sysroot;
   };
 
 in clangMulti

pkgs/development/compilers/llvm/rocm/default.nix

@@ -17,7 +17,6 @@ in rec {
       mkdir "$rsrc"
       ln -s "${cc}/lib/clang/$clang_version/include" "$rsrc"
       echo "-resource-dir=$rsrc" >> $out/nix-support/cc-cflags
-      echo "--gcc-toolchain=${stdenv.cc.cc}" >> $out/nix-support/cc-cflags
       echo "-Wno-unused-command-line-argument" >> $out/nix-support/cc-cflags
       rm $out/nix-support/add-hardening.sh
       touch $out/nix-support/add-hardening.sh

pkgs/top-level/all-packages.nix

@@ -9498,8 +9498,6 @@ in
       mkdir -p "$rsrc/lib"
       ln -s "${cc}/lib" "$rsrc/include"
       echo "-resource-dir=$rsrc" >> $out/nix-support/cc-cflags
-    '' + lib.optionalString (stdenv.targetPlatform.isLinux && !(stdenv.targetPlatform.useLLVM or false)) ''
-      echo "--gcc-toolchain=${gccForLibs}" >> $out/nix-support/cc-cflags
     '';
   };