Currently `cpuinfo.countProcessor` uses hard-coded `HW_AVAILCPU=25` for
both MacOS and BSD;
However,
[There is no HW_AVAILCPU on FreeBSD, NetBSD, and OpenBSD](
https://bugs.webkit.org/show_bug.cgi?id=132542)
Also, `HW_AVAILCPU` is undocmented in MacOS,
while `sysctlbyname("hw.logicalcpu",...)` is documented and used
by many other languages' implementations, like
[Haskell](https://gitlab.haskell.org/ghc/ghc/-/blob/master/rts/posix/OSThreads.c?ref_type=heads#L376)
---
This PR:
- use `importc` value instead of hard-coded values for `HW_*` macros.
- use "hw.logicialcpu" over undocumented HW_AVAILCPU.
- reduce 2 elements of `mib` array when calling `sysctl` as they're no
use.
e0e698be9a