During review of JDK-8322420 it was mentioned that some use-cases for the container code need the host active CPU count and not only the containerized value (which might be equal or less). I therefore, propose to make os::Linux::active_processor_count() public, which provides this info currently. It would be useful to report the host active cpus in JFR events too - if they differ to the containerized value - like we do for memory.

Some code that works around this issue currently, then needs to be cleaned up once this bug is fixed.