xcbuild: assorted fixes and cleanups

This is in preparation for the LLVM 4 upgrade (which gets more strict
about e.g., return false in xcbuild itself) and also for using xcbuild
more extensively in the Darwin stdenv bootstrap process, which is why I
killed the unnecessary gcc dependency in the toolchain. llvm-cov pretends
to be gcov anyway, so we're fine.

pkgs/development/tools/xcbuild/wrapper.nix

@@ -50,10 +50,10 @@ stdenv.mkDerivation {
     ln -s ${xcbuild}/Library/Xcode/Specifications $out/Library/Xcode/Specifications
 
     mkdir -p $out/Platforms/
-    ln -s ${platform} $out/Platforms/
+    ln -s ${platform} $out/Platforms/nixpkgs.platform
 
     mkdir -p $out/Toolchains/
-    ln -s ${toolchain} $out/Toolchains/
+    ln -s ${toolchain} $out/Toolchains/nixpkgs.xctoolahin
 
     wrapProgram $out/bin/xcodebuild \
       --add-flags "-xcconfig ${xcconfig}" \