[X86] Prefer locked stack op over mfence for seq_cst 64-bit stores on 32-bit targets

This is a follow on to D58632, with the same logic. Given a memory operation which needs ordering, but doesn't need to modify any particular address, prefer to use a locked stack op over an mfence.

Differential Revision: https://reviews.llvm.org/D61863

llvm-svn: 360649