RFR: JDK-8274434: move os::get_default_process_handle and os::dll_lookup to os_posix for POSIX platforms

[Matthias Baesken]

Hello, please review this small change.
The os::get_default_process_handle and os::dll_lookup functions are pretty similar across the POSIX platforms so they can be moved to os_posix.cpp.

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Commit messages:
 - JDK-8274434

Changes: https://git.openjdk.java.net/jdk/pull/5737/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5737&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274434
  Stats: 49 lines in 4 files changed: 15 ins; 34 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5737.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5737/head:pull/5737

PR: https://git.openjdk.java.net/jdk/pull/5737