cc-wrapper: useGccForLibs for libcxx == null

Since commit 11302dc ("clang, cc-wrapper: Move `--gcc-toolchain`
logic into CC wrapper") --gcc-toolchain was only passed i useGccForLibs
is true. However useGccForLibs is disabled if libcxx is null, which
results in the libcxxStdenv failing, since the linker can't find libgcc
while linking. Remove the condition check for libcxx == null, to let the
wrapper tests pass again.

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

@@ -62,7 +62,6 @@ let
     else "";
 
   useGccForLibs = isClang
-    && libcxx == null
     && !(stdenv.targetPlatform.useLLVM or false)
     && !(stdenv.targetPlatform.useAndroidPrebuilt or false)
     && gccForLibs != null;