[X86] Only define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 in 64-bit mode.

Summary:
This define should correspond to CMPXCHG16B being available which requires 64-bit mode.

I checked and gcc also seems to only define this in 64-bit mode.

Reviewers: RKSimon, spatel, efriedma, jyknight, jfb

Reviewed By: jfb

Subscribers: jfb, cfe-commits, llvm-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59287

llvm-svn: 356118