[X86] AVX512: Add non-temporal stores

Note that I followed the AVX2 convention here and didn't add LLVM intrinsics
for stores.  These can be generated with the nontemporal hint on LLVM IR
stores (see new test). The GCC builtins are lowered directly into nontemporal
stores.

<rdar://problem/17082571>

llvm-svn: 211176