[jdk17u-dev] RFR: 8339148: Make os::Linux::active_processor_count() public

[Goetz Lindenmaier]

I backport this for parity with 17.0.16-oracle.

Resolved, but probably clean anyways.

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Commit messages:
 - Backport bc269de452ba2c6072529c3201059b2039210238

Changes: https://git.openjdk.org/jdk17u-dev/pull/3439/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=3439&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339148
  Stats: 5 lines in 1 file changed: 2 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/3439.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/3439/head:pull/3439

PR: https://git.openjdk.org/jdk17u-dev/pull/3439