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

[Tom Rodriguez]

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.

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Commit messages:
 - Allow UseZGC with JVMCI and enable nmethod entry barrier support

Changes: https://git.openjdk.org/jdk/pull/11996/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11996&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299229
  Stats: 610 lines in 31 files changed: 335 ins; 149 del; 126 mod
  Patch: https://git.openjdk.org/jdk/pull/11996.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11996/head:pull/11996

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