[X86] Correct the value of MaxAtomicInlineWidth for pre-586 cpus

Use the new cx8 feature flag that was added to the backend to represent support for cmpxchg8b. Use this flag to set the MaxAtomicInlineWidth.

This also assumes all the cmpxchg instructions are enabled for CK_Generic which is what cc1 defaults to when nothing is specified.

Differential Revision: https://reviews.llvm.org/D59566

llvm-svn: 356709