powerpc: Use builtins for fls/flsl

Summary:
There's no need to use the fallback fls() and flsl() libkern functions
when the PowerISA includes instructions that already do the bulk of the
work. Take advantage of this through the GCC builtins builtin_clz()
and builtin_clzl().

Reviewed by: luporl
Differential Revision: https://reviews.freebsd.org/D22340