RFR: 8294729: [s390] Implement nmethod entry barriers

[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.

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Commit messages:
 - Fixup
 - Remove tmp register from s390 nmethod_entry_barrier. Use R0 & R1 implicitly
 - Invert condition
 - Add missing include to s390.ad
 - Change s390.ad register
 - Move s390.ad code to proper position
 - Remove uneeded #include
 - Incorporate MD's suggestions
 - Minor clean up
 - Change arg computation for vm-call in line with Martin's feedback. Remove redundant instr.
 - ... and 26 more: https://git.openjdk.org/jdk/compare/085949a1...0a683eee

Changes: https://git.openjdk.org/jdk/pull/10558/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10558&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294729
  Stats: 207 lines in 10 files changed: 198 ins; 0 del; 9 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