RFR: 8287325: AArch64: fix virtual threads with -XX:UseBranchProtection=pac-ret

[Nick Gasson]

The continuation free/thaw mechanism relies on being able to move thread stacks around in memory.  However when PAC is enabled on supported AArch64 CPUs, the saved LR on the stack contains a "pointer authentication code" signed with the stack pointer at the time the frame was created.  When a stack frame is relocated we need to re-sign the LR with the new stack pointer to ensure it authenticates successfully when the method returns.

Introduced `ContinuationHelper::return_pc_at()` to avoid directly reading the saved PC from the stack in shared code.  On AArch64 with PAC it enabled it strips the PAC from the address after reading it, on all other platforms it just loads the PC from the stack as before.

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Commit messages:
 - 8287325: AArch64: fix virtual threads with -XX:UseBranchProtection=pac-ret

Changes: https://git.openjdk.java.net/jdk/pull/9067/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9067&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8287325
  Stats: 80 lines in 16 files changed: 61 ins; 0 del; 19 mod
  Patch: https://git.openjdk.java.net/jdk/pull/9067.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/9067/head:pull/9067

PR: https://git.openjdk.java.net/jdk/pull/9067