kmp_lock.h: Fix VS2013 build after r271324

MSVC doesn't allow std::atomic<>s in a union since they don't have trivial
copy constructor. Replacing them with e.g. std::atomic_int works, but that
breaks the GCC build on Linux, because then calls to e.g. std::atomic_load_explicit
fail, as they expect a real std::atomic<> pointer.

Fixing this with an #ifdef to unbreak the build for now.

llvm-svn: 272271