RFR: 8329597: C2: Intrinsify Reference.clear

[Aleksey Shipilev]

[JDK-8240696](https://bugs.openjdk.org/browse/JDK-8240696) added the native method for `Reference.clear`. The original patch skipped intrinsification of this method, because we thought `Reference.clear` is not on a performance sensitive path. However, it shows up prominently on simple benchmarks that touch e.g. `ThreadLocal` cleanups. See the bug for an example profile with `RRWL` benchmarks.

Additional testing:
 - [x] Linux x86_64 server fastdebug, `all`
 - [ ] Linux AArch64 server fastdebug, `all`

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Commit messages:
 - Move the membar at the end
 - Revert C1 parts
 - Work

Changes: https://git.openjdk.org/jdk/pull/20139/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20139&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329597
  Stats: 132 lines in 7 files changed: 132 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20139.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20139/head:pull/20139

PR: https://git.openjdk.org/jdk/pull/20139