RFR: 8299312: Clean up BarrierSetNMethod [v2]

[Erik Österlund]

> The terminology in BarrierSetNMethod is not crisp. In platform code we talk about a per-nmethod "guard value", but on shared level we call the same value arm value or disarm value in different contexts. But it really depends on the value whether the nmethod is disarmed or armed. We should embrace the "guard value" terminology and lift it in to the shared code level.
> We also have more functionality than we need on platform level. The platform level only needs to know how to deoptimize, and how to set/get the guard value of an nmethod. The more specific functionality should be moved to the shared code and be expressed in terms of said setter/getter.

Erik Österlund 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 four additional commits since the last revision:

 - ARM support
 - Merge branch 'master' into 8299312_barrier_set_nmethod_cleanup
 - Fix Shenandoah build
 - 8299312: Clean up BarrierSetNMethod

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11774/files
  - new: https://git.openjdk.org/jdk/pull/11774/files/78afd161..e0b32db3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11774&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11774&range=00-01

  Stats: 9893 lines in 672 files changed: 5058 ins; 2615 del; 2220 mod
  Patch: https://git.openjdk.org/jdk/pull/11774.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11774/head:pull/11774

PR: https://git.openjdk.org/jdk/pull/11774