[XRay] Support AArch64 in Clang

This patch adds XRay support in Clang for AArch64 target.
This patch is one of a series:

LLVM: https://reviews.llvm.org/D26412
compiler-rt: https://reviews.llvm.org/D26413

Author: rSerge

Reviewers: rengolin, dberris

Subscribers: aemerson, cfe-commits, iid_iunknown

Differential Revision: https://reviews.llvm.org/D26415

llvm-svn: 287518