RFR: JDK-8299229: Allow UseZGC with JVMCI and enable nmethod entry barrier support [v4]

[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 pull request now contains seven commits:

 - Handle concurrent unloading
 - Merge branch 'master' into tkr-zgc
 - Add missing declaration
 - Replace NULL with nullptr in new code
 - Merge branch 'master' into tkr-zgc
 - Review fixes
 - Allow UseZGC with JVMCI and enable nmethod entry barrier support

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Changes: https://git.openjdk.org/jdk/pull/11996/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11996&range=03
  Stats: 538 lines in 27 files changed: 349 ins; 66 del; 123 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