[PATCH] [ARM64] Enable alignment control option in front-end for ARM64.

This patch is to get "-mno-unaligned-access" and "-munaligned-access"
work in front-end for ARM64 target.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208075 91177308-0d34-0410-b5e6-96231b3b80d8