[X86] _MM_ALIGN16 attribute support for non-windows targets

Summary:
This patch adds support for the _MM_ALIGN16 attribute on non-windows targets. This aligns Clang with ICC which supports the attribute on all targets.

Fixes PR28056

Reviewers: aaboud, echristo, cfe-commits, mkuper

Subscribers: zvi, mehdi_amini

Projects: #clang-c

Differential Revision: http://reviews.llvm.org/D21173

llvm-svn: 273095