RFR: 8294729: [s390] Implement nmethod entry barriers [v15]

[Tyler Steele]

> This draft PR implements native method barriers on s390. When complete, this will fix the build, and bring the other benefits of [JDK-8290025](https://bugs.openjdk.org/browse/JDK-8290025) to that platform.

Tyler Steele has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  Move nmethod_entry_barrier implementation

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10558/files
  - new: https://git.openjdk.org/jdk/pull/10558/files/72382f7b..dfbe898d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10558&range=14
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10558&range=13-14

  Stats: 71 lines in 6 files changed: 0 ins; 68 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/10558.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10558/head:pull/10558

PR: https://git.openjdk.org/jdk/pull/10558