Fix calculation of # CPU cores (FreeBSD/DragonFly)

[mneumann]

• On both FreeBSD and DragonFly, sysctl hw.ncpu returns the number of
  logical CPUs, NOT the number of physical CPU cores.

• FreeBSD has sysctl kern.smp.cores.

• DragonFly has sysctl's hw.cpu_topology_core_ids and
  hw.cpu_topology_phys_ids. Both multiplied together results in the
  number of cores.

• If kern.smp.cores (FreeBSD) or hw.cpu_topology_* (DragonFly) is
  not available, use hw.ncpu instead.

• For example on FreeBSD 12.2p3 running a i7-8565U:

  sysctl hw.ncpu # => 8
  sysctl kern.smp.cores # => 4

• For example on DragonFly 5.9-DEVELOPMENT with Xeon E5-1620:

  sysctl hw.ncpu # => 8
  sysctl hw.cpu_topology_phys_ids # => 1
  sysctl hw.cpu_topology_core_ids # => 4
  sysctl hw.cpu_topology_ht_ids # => 2

• I found this by running a HTTP benchmark on a FreeBSD system
  (i7-8565U, 4 cores / 8 logical CPUs), where setting --ponymaxthreads=4
  improved latency significantly over the default value "8" (hw.ncpu).
  And I was wondering why I could set --ponymaxthreads to 8, even so the
  documentation explicitly states that it cannot be set to anything
  above the number of CPU cores.

[ponylang-main]

Hi @mneumann,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 3707.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[mneumann]

@ponylang-main I added an entry in .release-notes. Thanks!

[SeanTAllen]

Thanks @mneumann. This will go out in the next ponyc release at the end of the month.