[safestack] Inline safestack pointer access when possible.

Summary:
This adds an -mllvm flag that forces the use of a runtime function call to
get the unsafe stack pointer, the same that is currently used on non-x86, non-aarch64 android.
The call may be inlined.

Reviewers: pcc

Subscribers: aemerson, kristof.beyls, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D37405

llvm-svn: 323259