clang: don't set machine flags for overridden target

We already did this for -march in 12b0e8ac74f8 ("clang: don't set
-march for overridden target"), but it should have been done for all
machine flags, for the same reason.

Example bug this fixes:

	nix-shell -E '
	  with import ./. {
	    crossSystem = {
	      system = "powerpc64le-linux";
	      gcc.cpu = "power10";
	    };
	  };
	  clangStdenv.mkDerivation { name = "test"; }
	' --run '$CC -target wasm32-unknown-unknown -c /dev/null'

Which previously failed with:

      clang: error: unsupported option '-mcpu=' for target 'wasm32-unknown-unknown'

0 files changed, 0 insertions, 0 deletions