[jdk21u-dev] RFR: 8339148: Make os::Linux::active_processor_count() public

[Goetz Lindenmaier]

I backport this for parity with 21.0.8-oracle

Trivial resolve, probably clean anyways.

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Commit messages:
 - Backport 12d060a255b9b783488714c6c2cb73a899d3f708

Changes: https://git.openjdk.org/jdk21u-dev/pull/1553/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1553&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/jdk21u-dev/pull/1553.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1553/head:pull/1553

PR: https://git.openjdk.org/jdk21u-dev/pull/1553