gcc: Always pass `--enable-shared` by default

I am actually a bit skeptical about this, but @matthewbauer makes the
case for this in
https://github.com/NixOS/nixpkgs/pull/107238#discussion_r546454453 and
I'm happy to go with it not being as in the loop on static linking stuff
as he is.

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

@@ -7,7 +7,12 @@
 , profiledCompiler ? false
 , langJit ? false
 , staticCompiler ? false
-, enableShared ? !stdenv.targetPlatform.isStatic
+, # N.B. the defult is intentionally not from an `isStatic`. See
+  # https://gcc.gnu.org/install/configure.html - this is about target
+  # platform libraries not host platform ones unlike normal. But since
+  # we can't rebuild those without also rebuilding the compiler itself,
+  # we opt to always build everything unlike our usual policy.
+  enableShared ? true
 , enableLTO ? true
 , texinfo ? null
 , perl ? null # optional, for texi2pod (then pod2man)

pkgs/development/compilers/gcc/4.8/default.nix

@@ -7,7 +7,12 @@
 , profiledCompiler ? false
 , langJit ? false
 , staticCompiler ? false
-, enableShared ? !stdenv.targetPlatform.isStatic
+, # N.B. the defult is intentionally not from an `isStatic`. See
+  # https://gcc.gnu.org/install/configure.html - this is about target
+  # platform libraries not host platform ones unlike normal. But since
+  # we can't rebuild those without also rebuilding the compiler itself,
+  # we opt to always build everything unlike our usual policy.
+  enableShared ? true
 , enableLTO ? true
 , texinfo ? null
 , perl ? null # optional, for texi2pod (then pod2man); required for Java

pkgs/development/compilers/gcc/4.9/default.nix

@@ -7,7 +7,12 @@
 , profiledCompiler ? false
 , langJit ? false
 , staticCompiler ? false
-, enableShared ? !stdenv.targetPlatform.isStatic
+, # N.B. the defult is intentionally not from an `isStatic`. See
+  # https://gcc.gnu.org/install/configure.html - this is about target
+  # platform libraries not host platform ones unlike normal. But since
+  # we can't rebuild those without also rebuilding the compiler itself,
+  # we opt to always build everything unlike our usual policy.
+  enableShared ? true
 , enableLTO ? true
 , texinfo ? null
 , perl ? null # optional, for texi2pod (then pod2man); required for Java

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

@@ -8,7 +8,12 @@
 , profiledCompiler ? false
 , langJit ? false
 , staticCompiler ? false
-, enableShared ? !stdenv.targetPlatform.isStatic
+, # N.B. the defult is intentionally not from an `isStatic`. See
+  # https://gcc.gnu.org/install/configure.html - this is about target
+  # platform libraries not host platform ones unlike normal. But since
+  # we can't rebuild those without also rebuilding the compiler itself,
+  # we opt to always build everything unlike our usual policy.
+  enableShared ? true
 , enableLTO ? true
 , texinfo ? null
 , flex

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

@@ -6,7 +6,12 @@
 , profiledCompiler ? false
 , langJit ? false
 , staticCompiler ? false
-, enableShared ? !stdenv.targetPlatform.isStatic
+, # N.B. the defult is intentionally not from an `isStatic`. See
+  # https://gcc.gnu.org/install/configure.html - this is about target
+  # platform libraries not host platform ones unlike normal. But since
+  # we can't rebuild those without also rebuilding the compiler itself,
+  # we opt to always build everything unlike our usual policy.
+  enableShared ? true
 , enableLTO ? true
 , texinfo ? null
 , perl ? null # optional, for texi2pod (then pod2man)

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

@@ -6,7 +6,12 @@
 , profiledCompiler ? false
 , langJit ? false
 , staticCompiler ? false
-, enableShared ? !stdenv.targetPlatform.isStatic
+, # N.B. the defult is intentionally not from an `isStatic`. See
+  # https://gcc.gnu.org/install/configure.html - this is about target
+  # platform libraries not host platform ones unlike normal. But since
+  # we can't rebuild those without also rebuilding the compiler itself,
+  # we opt to always build everything unlike our usual policy.
+  enableShared ? true
 , enableLTO ? true
 , texinfo ? null
 , perl ? null # optional, for texi2pod (then pod2man)

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

@@ -8,7 +8,12 @@
 , profiledCompiler ? false
 , langJit ? false
 , staticCompiler ? false
-, enableShared ? !stdenv.targetPlatform.isStatic
+, # N.B. the defult is intentionally not from an `isStatic`. See
+  # https://gcc.gnu.org/install/configure.html - this is about target
+  # platform libraries not host platform ones unlike normal. But since
+  # we can't rebuild those without also rebuilding the compiler itself,
+  # we opt to always build everything unlike our usual policy.
+  enableShared ? true
 , enableLTO ? true
 , texinfo ? null
 , perl ? null # optional, for texi2pod (then pod2man)