[jdk21u-dev] RFR: 8287325: AArch64: fix virtual threads with -XX:UseBranchProtection=pac-ret

[Aleksey Shipilev]

Clean backport to improve PAC-RET support with Virtual Threads.

Additional testing on AL2023 (has modern GCC) and Graviton 3 (has PAC extensions), `server fastdebug`:
 - [x] Seen `VMContinuations` disabled without the fix, kept enabled with the fix
 - [ ] `all` tests pass
 - [ ] `all` tests pass with configured `--enable-branch-protection`
 - [x] `all` tests pass with configured `--enable-branch-protection` and `-XX:UseBranchProtection=pac-ret`

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Commit messages:
 - Backport 481cfc798533f5b3adae7cc4a076a98b0b3f9737

Changes: https://git.openjdk.org/jdk21u-dev/pull/671/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=671&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8287325
  Stats: 236 lines in 23 files changed: 94 ins; 32 del; 110 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/671.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/671/head:pull/671

PR: https://git.openjdk.org/jdk21u-dev/pull/671