Fix other.test_bad_triple.

When we build native OSX executables with Clang, we must tell Clang which OSX SDK version we are targeting. Add a function CLANG_NATIVE_ARGS() that returns a list of compiler args that need to be appended when building native executables for the current platform, which for OSX automatically find an SDK to target. Make the machinery lazy and cached so that it's not invoked if native executables are not built, and that it's invoked only once when needed. Fixed test other.test_bad_triple on OSX.