[xray] Preserve x8 in trampoline on AArch64

Summary: Fixes an ABI violation in the trampoline code for AArch64 that causes the indirect result register to get overwritten if the XRay handler function is complex enough to use it.

Reviewers: MaskRay, dberris, johnislarry

Subscribers: kristof.beyls, danielkiss, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D78596