Implement the ffs and fls functions, and their longer counterparts, in
cpufunc, in terms of __builtin_ffs and the like, for arm64
architectures, and use those, rather than the simple libkern
implementations, in building arm64 kernels.

Tested by: greg_unrelenting.technology (earlier version)
Reviewed by: alc
Approved by: kib (mentor)
Differential Revision: https://reviews.freebsd.org/D20250