Fix bogomips calculation. Previously it was off by half. This was
verified under VMWare Fusion, comparing to what's reported under CentOS,
and by comparing numbers reported by linuxulator on T420 with a googled
up Linux cpuinfo (https://lkml.org/lkml/2011/11/29/116).

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20693