cc-wrapper: fix linking against GCC libs for non-GCC

Currently, clang++ statically links against libstdc++ instead of
dynamically links, because the -L path included in the cc-wrapper is
incorrect. The gccForLibs.lib output only contains the architecture
subdirectory if the target and host platform are not the same. (See
targetConfig set in gcc/<version>/default.nix and the gcc/builder.nix)

This fixes the incorrect linking by using the correct path for both the
cross and native cases. This also matches the cc_solib set above in
cc-wrapper/default.nix

Tested by compiling a simple cpp binary and noting that it now correctly
dynamically links against libstdc++, natively on x86_64 and arm64, as
well as x86_64 -> arm64 cross compilation.

Co-Authored-By: Sebastian Ullrich <sebasti@nullri.ch>

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

@@ -70,6 +70,8 @@ let
     && !(stdenv.targetPlatform.useAndroidPrebuilt or false)
     && !(stdenv.targetPlatform.isiOS or false)
     && gccForLibs != null;
+  gccForLibs_solib = getLib gccForLibs
+    + optionalString (targetPlatform != hostPlatform) "/${targetPlatform.config}";
 
   # older compilers (for example bootstrap's GCC 5) fail with -march=too-modern-cpu
   isGccArchSupported = arch:
@@ -311,7 +313,7 @@ stdenv.mkDerivation {
 
       echo "-B${gccForLibs}/lib/gcc/${targetPlatform.config}/${gccForLibs.version}" >> $out/nix-support/cc-cflags
       echo "-L${gccForLibs}/lib/gcc/${targetPlatform.config}/${gccForLibs.version}" >> $out/nix-support/cc-ldflags
-      echo "-L${gccForLibs.lib}/${targetPlatform.config}/lib" >> $out/nix-support/cc-ldflags
+      echo "-L${gccForLibs_solib}/lib" >> $out/nix-support/cc-ldflags
     ''
 
     # TODO We would like to connect this to `useGccForLibs`, but we cannot yet