[PartitionAlloc] Merge SpinLock into SpinningFutex.

Delete SpinLock, and implement it as a special-case of
SpinningMutex. This is done for two reasons:
- Unify implementations, since the constants used inside were already
  identical.
- Allow to choose at runtime rather than build-time which flavor to
  use. This is not done yet, but may be useful on some platforms.

Bug: 998048
Change-Id: Ia8cac347e1ce52743e7e264c09275a67339ec356
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3141444


Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/main@{#918751}