gcc: allow stripping gcc libraries

When cross compiling to the same kernel / arch combination, it is safe
to use strip of libraries. This happens when cross-compiling musl
programs. dontStrip is now set in each gcc compiler instead of in
gcc/builder.sh.

Fixes #75476

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

@@ -29,8 +29,8 @@
 , threadsCross ? null # for MinGW
 , crossStageStatic ? false
 , # Strip kills static libs of other archs (hence no cross)
-  stripped ? stdenv.hostPlatform == stdenv.buildPlatform
-          && stdenv.targetPlatform == stdenv.hostPlatform
+  stripped ? stdenv.hostPlatform.system == stdenv.buildPlatform.system
+          && stdenv.targetPlatform.system == stdenv.hostPlatform.system
 , gnused ? null
 , cloog # unused; just for compat with gcc4, as we override the parameter on some places
 , buildPackages