Fix __attribute__((force_align_arg_pointer)) misalignment bug

Fix __attribute__((force_align_arg_pointer)) misalignment bug

The force_align_arg_pointer attribute was using a hardcoded 16-byte
alignment value which in combination with -mstack-alignment=32 (or
larger) would produce a misaligned stack which could result in crashes
when accessing stack buffers using aligned AVX load/store instructions.

Fix the issue by using the "stackrealign" function attribute instead
of using a hardcoded 16-byte alignment.

Patch By: Gramner

Differential Revision: https://reviews.llvm.org/D45812

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@330331 91177308-0d34-0410-b5e6-96231b3b80d8