[CodeGen][x86_64] Enable 'force_align_arg_pointer' attribute at x86_64

This attribute is useful in OS development when we jump from 32 to 64 bit
code and expect that 64bit function forces correct stack alignment.

Related discussion: http://lists.llvm.org/pipermail/cfe-dev/2017-June/054358.html

Patch By: anatol.pomozov (anatol.pomozov@gmail.com)

Differential Revision:https://reviews.llvm.org/D36272


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@312173 91177308-0d34-0410-b5e6-96231b3b80d8