RFR: 8287818: Shenandoah: adapt nmethod arming from Loom [v3]

[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

Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:

  Add assertion

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/9048/files
  - new: https://git.openjdk.org/jdk/pull/9048/files/b4ef6d63..b496a508

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9048&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9048&range=01-02

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9048.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9048/head:pull/9048

PR: https://git.openjdk.org/jdk/pull/9048