RFR: 8299312: Clean up BarrierSetNMethod

[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.

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Commit messages:
 - Fix Shenandoah build
 - 8299312: Clean up BarrierSetNMethod

Changes: https://git.openjdk.org/jdk/pull/11774/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11774&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299312
  Stats: 159 lines in 26 files changed: 10 ins; 73 del; 76 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