RFR: 8288023: AArch64: disable PAC-RET when preview is enabled

[Nick Gasson]

PAC-RET is not currently compatible with the continuation freeze/thaw mechanism used by virtual threads, and additionally
`-XX:UseBranchProtection=pac-ret` enables `PreserveFramePointer` which prevents the use of the continuation thaw fast-path (about 15% slower for the "Skynet" JMH benchmark).  So for now just disable PAC when `--enable-preview` was also passed.

See some earlier discussion here: #9067

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Commit messages:
 - 8288023: AArch64: disable PAC-RET when preview is enabled

Changes: https://git.openjdk.java.net/jdk/pull/9102/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9102&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288023
  Stats: 7 lines in 1 file changed: 5 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/9102.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/9102/head:pull/9102

PR: https://git.openjdk.java.net/jdk/pull/9102