[Atomic][X8664] set max atomic inline width according to the target

This is to fix PR31620. MaxAtomicInlineWidth is set to 128 for x86_64. However
for target without cx16 support, 128 atomic operation will generate __sync_*
libcalls. The patch set MaxAtomicInlineWidth to 64 if the target doesn't support
cx16.

Differential Revision: https://reviews.llvm.org/D38046

llvm-svn: 313992