[X86][inline-asm] Add support for MS 'EVEN' directive

MS assembly syntax provide us with the 'EVEN' directive as a synonymous to at&t '.even'.
This patch include the (small, simple) changes need to allow it.

Test is provided at the following (clang-side) review:
https://reviews.llvm.org/D27418

Differential Revision: https://reviews.llvm.org/D27417

llvm-svn: 299453