RFR: 8321371: SpinPause() not implemented for bsd_aarch64/macOS

[Fredrik Bredberg]

The SpinPause() function only returns 0 on bsd_aarch64 (i.e. macOS)

This PR initially meant to implement SpinPause() for macOS on AArch64 by copying the source from linux_aarch64, but after having some internal discussions, it seems like the most reasonable thing to do is to implement SpinPause() using a single inline yield instruction.

Tested successfully on macosx-aarch64 tier1-tier5.

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Commit messages:
 - Merge branch 'master' into 8321371_spinpause_for_bsd_aarch64
 - SpinPause() now uses hard-coded yield instruction on bsd_aarch64/macOS
 - 8321371: SpinPause() not implemented for bsd_aarch64/macOS

Changes: https://git.openjdk.org/jdk/pull/16994/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16994&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321371
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16994.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16994/head:pull/16994

PR: https://git.openjdk.org/jdk/pull/16994