RFR: 8282360: Merge POSIX implementations of ThreadCritical

[Harold Seigel]

Please review this small change to merge the Posix (aix, bsd, linux) implementations of ThreadCritical into a single file named src/hotspot/os/posix/threadCritical_posix.cpp.  The change was tested by running Mach5 tiers 1-2 on Linux, Mac OS, and Windows, and Mach5 tiers 3-5 on Linux x64.

Thanks, Harold

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Commit messages:
 - 8282360: Merge POSIX implementations of ThreadCritical

Changes: https://git.openjdk.java.net/jdk/pull/7622/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7622&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282360
  Stats: 125 lines in 3 files changed: 1 ins; 123 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7622.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7622/head:pull/7622

PR: https://git.openjdk.java.net/jdk/pull/7622