illums compat: use flsl/flsll for highbit/highbit64

Do that only when when fast inline versions are available.
At the moment that can be the case only in the kernel and not for all
platforms.

The original code uses the binary search and that's kept as a fallback.
This is a micro optimization.

Differential Revision: https://reviews.freebsd.org/D2839
Reviewed by: delphij, mahrens, mav
MFC after: 17 days