Add entries for Expanding atomic intrinsics to libcalls. Just a placeholder

Add entries for Expanding atomic intrinsics to libcalls. Just a placeholder
for the moment. The implementation of the libcall will follow.

Currently, the llvm-gcc knows when the intrinsics can be correctly handled by
the back end and only generates them in those cases, issuing libcalls directly
otherwise. That's too much coupling. The intrinsics should always be
generated and the back end decide how to handle them, be it with a libcall,
inline code, or whatever. This patch is a step in that direction.

rdar://8097623

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@106227 91177308-0d34-0410-b5e6-96231b3b80d8