Use the retguard macros from asm.h to protect the function.
ok mortimer kettenis

Provide an optimized implementation of ffs(3) in the kernel on
arm64/powerpc/powerpc64, making use of the count leading zeros
instruction.

powerpc testing by cwen@; ok kettenis@ deraadt@

Provide an optimized implementation of ffs(3) in the kernel on
arm64/powerpc/powerpc64, making use of the count leading zeros
instruction.

powerpc testing by cwen@; ok kettenis@ deraadt@