RFR: 8339148: Make os::Linux::active_processor_count() public

[Severin Gehwolf]

Please review this Linux-only change which makes `os::Linux::active_processor_count()` public (from currently protected). This allows us to clean up some `friend class` usages and will also make some container related patches less clunky.

Testing:
- [ ] GHA
- [x] Container tests on linux x86_64

Thoughts?

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Commit messages:
 - 8339148: Make os::Linux::active_processor_count() public

Changes: https://git.openjdk.org/jdk/pull/20833/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20833&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339148
  Stats: 4 lines in 1 file changed: 1 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20833.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20833/head:pull/20833

PR: https://git.openjdk.org/jdk/pull/20833