cmd/compile, cmd/internal/obj/ppc64: use LR for indirect calls
On PPC64, indirect calls can be made through LR or CTR. Currently
both are used. This CL changes it to always use LR.
For async preemption, to return from the injected call, we need
an indirect jump back to the PC we preeempted. This jump can be
made through LR or CTR. So we'll have to clobber either LR or CTR.
Currently, LR is used more frequently. In particular, for a leaf
function, LR is live throughout the function. We don't want to
make leaf functions nonpreemptible. So we choose CTR for the call
injection. For code sequences that use CTR, if it is ok to use
another register, change it to.
Plus, it is a call so it will clobber LR anyway. It doesn't need
to also clobber CTR (even without preemption).