Move disabling of gccForLibs for iOS to build-support cc-wrapper

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

@@ -65,6 +65,7 @@ let
     && libcxx == null
     && !(stdenv.targetPlatform.useLLVM or false)
     && !(stdenv.targetPlatform.useAndroidPrebuilt or false)
+    && !(stdenv.targetPlatform.isiOS or false)
     && gccForLibs != null;
 
   # older compilers (for example bootstrap's GCC 5) fail with -march=too-modern-cpu

pkgs/top-level/all-packages.nix

@@ -11870,7 +11870,6 @@ in
     # Can only do this is in the native case, otherwise we might get infinite
     # recursion if `targetPackages.stdenv.cc.cc` itself uses `gccForLibs`.
       then targetPackages.stdenv.cc.cc
-    else if targetPlatform.isiOS then null
     else gcc.cc;
 
   libstdcxx5 = callPackage ../development/libraries/gcc/libstdc++/5.nix { };