RFR: JDK-8295710 : unify os::dll_file_extension

[Matthias Baesken]

The os::dll_file_extension() function could be unified for the different OS platforms.

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Commit messages:
 - JDK-8295710

Changes: https://git.openjdk.org/jdk/pull/10782/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10782&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295710
  Stats: 11 lines in 5 files changed: 2 ins; 9 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10782.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10782/head:pull/10782

PR: https://git.openjdk.org/jdk/pull/10782