This CL adds support of call injection and async preemption on
PPC64.

For the injected call to return to the preempted PC, we have to
clobber either LR or CTR. For reasons mentioned in previous CLs,
we choose CTR. Previous CLs have marked code sequences that use
CTR async-nonpreemtible.

Change-Id: Ia642b5f06a890dd52476f45023b2a830c522eee0
Reviewed-on: https://go-review.googlesource.com/c/go/+/203824
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>