RFR: 8287818: Shenandoah: adapt nmethod arming from Loom

[Zhengyu Gu]

Loom implemented nmethod arming mechanism, which is very similar to Shenandoah's. Shenandoah can use it.

Test:

- [x] hotspot_gc_shenandoah on Linux x86_64 and Windows x64
- [x] tier1 with Shenandoah GC on Linux x86_64 and Windows x64 
- [x] hotspot_gc_shenandoah on AArch64

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Commit messages:
 - 8287818: Shenandoah: adapt nmethod arming from Loom
 - Merge branch 'master' into migrate_nmethod_arming_shenandoah
 - v0

Changes: https://git.openjdk.java.net/jdk/pull/9048/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9048&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8287818
  Stats: 53 lines in 5 files changed: 4 ins; 41 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/9048.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/9048/head:pull/9048

PR: https://git.openjdk.java.net/jdk/pull/9048