clang_11: Fix RISC-V builds for compiler-rt. (#135718)

Closes: #135628

pkgs/development/compilers/llvm/11/compiler-rt/default.nix

@@ -112,5 +112,8 @@ stdenv.mkDerivation {
     # "All of the code in the compiler-rt project is dual licensed under the MIT
     # license and the UIUC License (a BSD-like license)":
     license = with lib.licenses; [ mit ncsa ];
+    # compiler-rt requires a Clang stdenv on 32-bit RISC-V:
+    # https://reviews.llvm.org/D43106#1019077
+    broken = stdenv.hostPlatform.isRiscV && stdenv.hostPlatform.is32bit && !stdenv.cc.isClang;
   };
 }

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

@@ -227,7 +227,7 @@ let
 
     compiler-rt-libc = callPackage ./compiler-rt {
       inherit llvm_meta;
-      stdenv = if (stdenv.hostPlatform.useLLVM or false) || (stdenv.hostPlatform.isDarwin && stdenv.hostPlatform.isAarch64)
+      stdenv = if (stdenv.hostPlatform.useLLVM or false) || (stdenv.hostPlatform.isDarwin && stdenv.hostPlatform.isAarch64) || (stdenv.hostPlatform.isRiscV && stdenv.hostPlatform.is32bit)
                then overrideCC stdenv buildLlvmTools.clangNoCompilerRtWithLibc
                else stdenv;
     };
@@ -240,7 +240,7 @@ let
     };
 
     # N.B. condition is safe because without useLLVM both are the same.
-    compiler-rt = if stdenv.hostPlatform.isAndroid || (stdenv.hostPlatform.isDarwin && stdenv.hostPlatform.isAarch64)
+    compiler-rt = if stdenv.hostPlatform.isAndroid || (stdenv.hostPlatform.isDarwin && stdenv.hostPlatform.isAarch64) || (stdenv.hostPlatform.libc == "newlib")
       then libraries.compiler-rt-libc
       else libraries.compiler-rt-no-libc;