Add support for -mcx16, and predefine __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 when
it is enabled. Also enable it on the same architectures that GCC does.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192045 91177308-0d34-0410-b5e6-96231b3b80d8