RFR: 8294729: [s390] Implement nmethod entry barriers [v14]

[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:

  Changes
  
  - Adds labels to registers used in c2i_entry_barrier
  - Removes use of R2
  - Removes erroneous uses of R0
  - Adds nm->c2i_entry_barrier in gen_i2c2i_adapters

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/10558/files
  - new: https://git.openjdk.org/jdk/pull/10558/files/a744ce34..72382f7b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10558&range=13
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10558&range=12-13

  Stats: 23 lines in 3 files changed: 9 ins; 1 del; 13 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