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

[Tom Rodriguez]

On Fri, 13 Jan 2023 20:02:26 GMT, Tom Rodriguez <never at openjdk.org> wrote:

> This exposes the required ZGC values to JVMCI and adds support for nmethod entry barriers.  The ZGC support is straightforward but the nmethod entry barrier required some reworking to fit better into JVMCI usage.  I also removed the epoch based barrier since it was no longer used with simplified the assumptions on the JVMCI side.  There is also a minor loom related fix to support post call nops included.  I could move that into a separate PR if that would be preferred.

I've dropped the removal of the epoch stuff and exposed the epoch addr for use by code generation.

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PR: https://git.openjdk.org/jdk/pull/11996