Add support for ARM64 unwinding to `unwind_util`.

Note that this change will:
* Not enable Stack Sampled Metrics (`ThreadProfiler`) on ARM64
  yet: `ThreadProfiler` has its own platform configuration and
  is enabled only for ARM32 currently.
* Enable ARM64 unwinding for Background Tracing
  (`TracingSamplerProfiler`). I've tested this with local
  traces, and it appears to be working well.

Bug: 1392158
Change-Id: I75fa1fdd9c0fa210ee13dc4c3be0fc2a6a7a46db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4043522


Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: Tushar Agarwal <agarwaltushar@google.com>
Cr-Commit-Position: refs/heads/main@{#1075566}