RFR: 8272811: Document the affects of building with _GNU_SOURCE in os_posix.hpp

[David Holmes]

Please review this trivial code documentation enhancement based on discussions here:

http://mail.openjdk.java.net/pipermail/jdk-dev/2021-August/005946.html

Thanks,
David

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Commit messages:
 - Fixed trailing whitespace
 - 8272811: Document the affects of building with _GNU_SOURCE in os_posix.hpp

Changes: https://git.openjdk.java.net/jdk/pull/5212/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5212&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272811
  Stats: 9 lines in 1 file changed: 9 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5212.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5212/head:pull/5212

PR: https://git.openjdk.java.net/jdk/pull/5212