RFR: 8369631: Assess and remedy any unsafe usage of the sr_semaphore Semaphore in the Posix signal code

[David Holmes]

Replace the statically allocated "semaphore" with a pointer and lazily initialize it. I chose this approach as we already have a second Semaphore declared that way.

Testing:
- tiers 1-3 sanity

Thanks.

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Commit messages:
 - 8369631: Assess and remedy any unsafe usage of the sr_semaphore Semaphore in the Posix signal code

Changes: https://git.openjdk.org/jdk/pull/27762/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27762&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369631
  Stats: 15 lines in 1 file changed: 6 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/27762.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27762/head:pull/27762

PR: https://git.openjdk.org/jdk/pull/27762