RFR: 8341967: Unify os::current_file_offset and os::seek_to_file_offset across posix platforms

[Matthias Baesken]

The methods os::current_file_offset and os::seek_to_file_offset are the same across the POSIX platforms so they can be moved to os_posix.

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Commit messages:
 - JDK-8341967

Changes: https://git.openjdk.org/jdk/pull/21471/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21471&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341967
  Stats: 40 lines in 4 files changed: 10 ins; 30 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/21471.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21471/head:pull/21471

PR: https://git.openjdk.org/jdk/pull/21471