RFR: JDK-8327444: simplify RESTARTABLE macro usage in JDK codebase [v2]

[Matthias Baesken]

> We define the RESTARTABLE macro again and again at a lot of places in the JDK native codebase. This could be centralized to avoid repeating it again and again !

Matthias Baesken has updated the pull request incrementally with one additional commit since the last revision:

  introduce unix jni_util_md.h

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18132/files
  - new: https://git.openjdk.org/jdk/pull/18132/files/014ab1fd..f30a189d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18132&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18132&range=00-01

  Stats: 68 lines in 19 files changed: 52 ins; 7 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/18132.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18132/head:pull/18132

PR: https://git.openjdk.org/jdk/pull/18132