RFR: JDK-8273979: move some os time related functions to os_posix for POSIX platforms

[Matthias Baesken]

There are a couple of time related functions in os_<platform> for Linux, BSD+Mac and AIX that are pretty similar (or identical) across the platforms and can be centralized in os_posix .
While doing so, I noticed that  os::supports_vtime()   returns always true , on Posix and also on Windows, so we might remove it completely if this prefered ? 

Best regards , Matthias

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Commit messages:
 - JDK-8273979

Changes: https://git.openjdk.java.net/jdk/pull/5582/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5582&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273979
  Stats: 255 lines in 4 files changed: 65 ins; 187 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5582.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5582/head:pull/5582

PR: https://git.openjdk.java.net/jdk/pull/5582