RFR: 8251438: Issues with our POSIX set_signal_handler()

[Gerard Ziemski]

This is a fix for a potential issue involving "The storage occupied by sa_handler and sa_sigaction may overlap, and a conforming application shall not use both simultaneously." https://pubs.opengroup.org/onlinepubs/009695399/functions/sigaction.html, when we in fact do assume different storages.

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Commit messages:
 - update to latest
 - fix potential sa_handler/sa_sigaction overlap storage issue

Changes: https://git.openjdk.java.net/jdk/pull/1680/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1680&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251438
  Stats: 26 lines in 1 file changed: 11 ins; 10 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1680.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1680/head:pull/1680

PR: https://git.openjdk.java.net/jdk/pull/1680