gcc{6,7,8,9,10,11}: fix cross-compiler build on x86_64-darwin

Building a cross-compiler fails due to register storage class specifier
errors when building with clang 16 due to its defaulting to C++17.
Downgrading the error allows the older cross-compilers to build.

(cherry picked from commit f2a7764)

pkgs/development/compilers/gcc/default.nix

@@ -414,7 +414,11 @@ lib.pipe ((callFile ./common/builder.nix {}) ({
     badPlatforms = if targetPlatform != hostPlatform then [ "aarch64-darwin" ] else [ ];
   };
 } // optionalAttrs is7 {
-  env.NIX_CFLAGS_COMPILE = lib.optionalString (stdenv.cc.isClang && langFortran) "-Wno-unused-command-line-argument";
+  NIX_CFLAGS_COMPILE = lib.optionalString (stdenv.cc.isClang && langFortran) "-Wno-unused-command-line-argument"
+    # Downgrade register storage class specifier errors to warnings when building a cross compiler from a clang stdenv.
+    + lib.optionalString (stdenv.cc.isClang && targetPlatform != hostPlatform) " -Wno-register";
+} // optionalAttrs (!is7 && !atLeast12 && stdenv.cc.isClang && targetPlatform != hostPlatform) {
+  NIX_CFLAGS_COMPILE = "-Wno-register";
 } // lib.optionalAttrs (!atLeast10 && stdenv.targetPlatform.isDarwin) {
   # GCC <10 requires default cctools `strip` instead of `llvm-strip` used by Darwin bintools.
   preBuild = ''