[X86] Bump Darwin MaxVectorAlign to 64 when AVX512 is enabled.

Without this, 64-byte vector types (__m512), specified to be 64-byte
aligned in the AVX512 draft SysV ABI, will only be 32-byte aligned.

This is analoguous to AVX, for which we accept 32-byte max alignment.

Differential Revision: http://reviews.llvm.org/D10724

llvm-svn: 246230