Add Expand-to-libcall support for additional atomics. This covers the usual

Add Expand-to-libcall support for additional atomics. This covers the usual
entries used by llvm-gcc. *_[U]MIN and such can be added later if needed.

This enables the front ends to simplify handling of the atomic intrinsics by
removing the target-specific decision about which targets can handle the
intrinsics.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@106321 91177308-0d34-0410-b5e6-96231b3b80d8