RFR: JDK-8299229: [JVMCI] add support for UseZGC [v8]

[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 incrementally with one additional commit since the last revision:

  Review comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11996/files
  - new: https://git.openjdk.org/jdk/pull/11996/files/a0dae2be..a29d7d84

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11996&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11996&range=06-07

  Stats: 141 lines in 10 files changed: 61 ins; 62 del; 18 mod
  Patch: https://git.openjdk.org/jdk/pull/11996.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/11996/head:pull/11996

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