Adapt SYS.h to use retguard macros from asm.h, so that generated system
calls are guarded. Adapt the first few hand-written functions to this
model (a few remain)
ok kettenis mortimer

Provide an optimized implementation of ffs(3) in libc on
aarch64/powerpc/powerpc64, making use of the count leading
zeros instruction. Also add a brief regression test.
ok deraadt@ kettenis@