[compiler-rt] Do not redefine builtins on Windows

Build bots are still failing, and getting it to work on Windows should
be done in a separate patch, should this even be technically feasible.

 | lld-link: error:
 | stage2_win_x64/obj/compiler-rt/lib/asan/asan_shared_library.asan_activation.obj:
 | memcpy should not refer to special section 0