[XRay] Support AArch64 in LLVM

This patch adds XRay support in LLVM for AArch64 targets.
This patch is one of a series:

Clang: https://reviews.llvm.org/D26415
compiler-rt: https://reviews.llvm.org/D26413

Author: rSerge

Reviewers: rengolin, dberris

Subscribers: amehsan, aemerson, llvm-commits, iid_iunknown

Differential Revision: https://reviews.llvm.org/D26412

llvm-svn: 287209