RFR: 8252324: Signal related code should be shared among POSIX platforms

[Gerard Ziemski]

This is a fresh start for https://github.com/openjdk/jdk/pull/157

Please review this change that refactors common POSIX code into a separate
file.

Currently there appears to be quite a bit of duplicated code among POSIX
platforms, which makes it difficult to apply single fix to the signal code.
With this fix, we will only need to touch single file for common POSIX
code fixes from now on.

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### Progress
- [x] Change must not contain extraneous whitespace
- [x] Commit message must refer to an issue
- [ ] Change must be properly reviewed



### Download
`$ git fetch https://git.openjdk.java.net/jdk pull/497/head:pull/497`
`$ git checkout pull/497`

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Commit messages:
 - Address David's review issues
 - Revert "Add AIX specific SA code"
 - Add AIX specific SA code
 - Remove leftover AIX signal code
 - removed white spaces
 - Refactored common POSIX signal code into seperate file

Changes: https://git.openjdk.java.net/jdk/pull/497/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=497&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252324
  Stats: 5361 lines in 21 files changed: 1739 ins; 3529 del; 93 mod
  Patch: https://git.openjdk.java.net/jdk/pull/497.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/497/head:pull/497

PR: https://git.openjdk.java.net/jdk/pull/497