[XRay][ARM32][AArch64] Fix unstable FDR tests on weak-ordering CPUs

Summary: Change from `compare_exchange_weak()` to `compare_exchange_strong()` where appropriate, because on ARM ( http://lab.llvm.org:8011/builders/clang-cmake-armv7-a15-full/builds/3190 , http://lab.llvm.org:8011/builders/clang-cmake-armv7-a15-full/builds/3191 ) and AArch64 ( http://lab.llvm.org:8011/builders/clang-cmake-aarch64-42vma/builds/3900 ) it fails even in single-threaded scenarios.

Reviewers: dberris, rengolin

Reviewed By: rengolin

Subscribers: aemerson, llvm-commits, iid_iunknown

Differential Revision: https://reviews.llvm.org/D29286

llvm-svn: 293505