build-support/cc-wrapper: add libstdc++fs into default library path f…

…or clang

After NixOS#210004 `usbmuxd2` started
failing to build as:

    usbmuxd2-unstable> .../ld: cannot find -lstdc++fs: No such file or directory
    usbmuxd2-unstable> clang-11: error: linker command failed with exit code 1 (use -v to see invocation)

This started happening because NixOS#210004 exposed a long-standing bug in
`gcc` derivation: `cc.lib` is missing `libstdc++fs` library:

    $ find $(nix-build --no-link -A stdenv.cc.cc.lib) | fgrep libstdc | unnix

    /<<NIX>>/gcc-11.3.0-lib/lib/libstdc++fs.la

    /<<NIX>>/gcc-11.3.0-lib/lib/libstdc++.la
    /<<NIX>>/gcc-11.3.0-lib/lib/libstdc++.so.6.0.29
    /<<NIX>>/gcc-11.3.0-lib/lib/libstdc++.so
    /<<NIX>>/gcc-11.3.0-lib/lib/libstdc++.so.6

It was not moved from `cc.out` output:

    $ find $(nix-build --no-link -A stdenv.cc.cc) | fgrep libstdc | unnix
    /<<NIX>>/gcc-11.3.0/lib/libstdc++.a
    /<<NIX>>/gcc-11.3.0/lib/libstdc++fs.a

This change adds `cc` library lookup path back to `staging-next` until
`gcc` is fixed.`

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

@@ -321,6 +321,11 @@ stdenv.mkDerivation {
                       && !(stdenv.targetPlatform.useLLVM or false)
                       && gccForLibs != null) ''
       echo "--gcc-toolchain=${gccForLibs}" >> $out/nix-support/cc-cflags
+
+      # Pull in 'cc.out' target to get 'libstdc++fs.a'. It should be in
+      # 'cc.lib'. But it's a gcc package bug.
+      # TODO(trofi): remove once gcc is fixed to move libraries to .lib output.
+      echo "-L${gccForLibs}/${optionalString (targetPlatform != hostPlatform) "/${targetPlatform.config}"}/lib" >> $out/nix-support/cc-ldflags
     ''
 
     ##