armv6/legacy: optimize cpu_getcount performance

Use nanotime instread of binuptime.
This change is for old and legacy platforms and does not impact any armv7 and later.
Might be MFCed to 13.0 once armv6 support is finally dropped in 14.0

Sponsored by: Stormshield
Reviewed by: mw
Obtained from: Semihalf
Differential revision: https://reviews.freebsd.org/D33719