RFR: 8361376: Regressions 1-6% in several Renaissance in 26-b4 only MacOSX aarch64 [v3]

[Dean Long]

> This PR removes the recently added lock around set_guard_value, using instead Atomic::cmpxchg to atomically update bit-fields of the guard value.  Further, it takes a fast-path that uses the previous direct store when at a safepoint.  Combined, these changes should get us back to almost where we were before in terms of overhead.  If necessary, we could go even further and allow make_not_entrant() to perform a direct byte store, leaving 24 bits for the guard value.

Dean Long has updated the pull request incrementally with one additional commit since the last revision:

  Fix PPC64

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26399/files
  - new: https://git.openjdk.org/jdk/pull/26399/files/e05605eb..a06b3446

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26399&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26399&range=01-02

  Stats: 24 lines in 2 files changed: 11 ins; 10 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/26399.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26399/head:pull/26399

PR: https://git.openjdk.org/jdk/pull/26399