[jdk21u-dev] RFR: 8317575: AArch64: C2_MacroAssembler::fast_lock uses rscratch1 for cmpxchg result

[Boris Ulasevich]

This is a clean backport to avoid a potentially rscratch1 register clobbering in C2_MacroAssembler::fast_lock.

Testing: jtreg tier1-3 on linux-aarch64.

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Commit messages:
 - Backport 387504c9e4b93d162dcef7c90c57c27295858d2e

Changes: https://git.openjdk.org/jdk21u-dev/pull/941/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=941&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317575
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/941.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/941/head:pull/941

PR: https://git.openjdk.org/jdk21u-dev/pull/941