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

[Ron Pressler]

On Thu, 9 Jun 2022 08:26:38 GMT, Nick Gasson <ngasson at openjdk.org> wrote:

> 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

@nick-arm If you integrate over the next couple of hours this will make it in before the 19 fork.

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