[X86] Add CMPXCHG8B feature flag. Set it for all CPUs except i386/i486...

[X86] Add CMPXCHG8B feature flag. Set it for all CPUs except i386/i486 including 'generic'. Disable use of CMPXCHG8B when this flag isn't set.

CMPXCHG8B was introduced on i586/pentium generation.

If its not enabled, limit the atomic width to 32 bits so the AtomicExpandPass will expand to lib calls. Unclear if we should be using a different limit for other configs. The default is 1024 and experimentation shows that using an i256 atomic will cause a crash in SelectionDAG.

Differential Revision: https://reviews.llvm.org/D59576

llvm-svn: 356631