RFR: 8294729: [s390] Implement nmethod entry barriers [v6]

[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 updated the pull request incrementally with one additional commit since the last revision:

  Add missing imports

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10558/files
  - new: https://git.openjdk.org/jdk/pull/10558/files/4a403b48..ae6c4c34

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10558&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10558&range=04-05

  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 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