RFR: JDK-8299229: Allow UseZGC with JVMCI and enable nmethod entry barrier support [v2]

[Tom Rodriguez]

On Tue, 25 Apr 2023 13:00:48 GMT, Erik Österlund <eosterlund at openjdk.org> wrote:

>> We've been happily running without them for a long time which is why we're just now getting around to supporting them.  Are we sitting on a ticking time bomb?  I do want to make them required as of 21 but maybe we could defer that to a future commit once my Graal changes to support the entry barriers has landed?
>
> Yeah unfortunately it is a ticking time bomb. Will you fix it for 21?

Yes I'll fix this here and also update our labsjdk to require it now.  Graal will now emit them for all GCs.  This code was really there to deal with the transition period where we needed a working JDK but Graal didn't support it yet.

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PR Review Comment: https://git.openjdk.org/jdk/pull/11996#discussion_r1176673831