[XRay][AArch64] Attempt to fix unstable test XRay-aarch64-linux::patching-unpatching.cc

Summary: Currently test XRay-aarch64-linux::patching-unpatching.cc sometimes passes, sometimes fails. This is an attempt to fix it by handling better the situations when both `__arm__` and `__aarch64__` are defined.

Reviewers: dberris, rengolin

Subscribers: llvm-commits, iid_iunknown, aemerson, rengolin, dberris

Differential Revision: https://reviews.llvm.org/D27421

llvm-svn: 288729