Implement the ffs and fls functions, and their longer counterparts, in
cpufunc, in terms of __builtin_ffs and the like, for arm32 v6 and v7
architectures, and use those, rather than the simple libkern
implementations, in building arm32 kernels.

Reviewed by: manu
Approved by: kib, markj (mentors)
Tested by: iz-rpi03_hs-karlsruhe.de, mikael.urankar_gmail.com, ian
Differential Revision: https://reviews.freebsd.org/D20412