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

[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.

Tom Rodriguez has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains one additional commit since the last revision:

  Allow UseZGC with JVMCI and enable nmethod entry barrier support

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11996/files
  - new: https://git.openjdk.org/jdk/pull/11996/files/bbc7df3b..d3ae96cb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11996&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11996&range=00-01

  Stats: 3429 lines in 495 files changed: 1184 ins; 681 del; 1564 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