RFR: 8307926: Support byte-sized atomic bitset operations

[Kim Barrett]

Please review this change to add support Atomic byte-sized bitset operations.
The existing default implementation using cmpxchg already (by accident) works
on such values, because cmpxchg supports byte-sized operations.  Any future
platform specializations of these operations will need to deal with this size
too.

The changes consist of updating the requirements documentation for the
operations and adding gtests for the newly supported size.

Testing:
mach5 tier1-3
GHA testing

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Commit messages:
 - support bitops on bytes

Changes: https://git.openjdk.org/jdk/pull/13952/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13952&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307926
  Stats: 17 lines in 2 files changed: 14 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/13952.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13952/head:pull/13952

PR: https://git.openjdk.org/jdk/pull/13952