Power8 inline assembly fixes

Power8 inline assembly fixes

Quoting patch author amodra from #1078
Lots of issues here.
- The vsx regs weren't listed as clobbered.
- Poor choice of vsx regs, which along with the lack of clobbers led to
  trashing v0..v21 and fr14..fr23.  Ideally you'd let gcc choose all
  temp vsx regs, but asms currently have a limit of 30 i/o parms.
- Other regs were clobbered unnecessarily, seemingly in an attempt to
  clobber inputs, with gcc-7 complaining about the clobber of r2.
  (Changed inputs should be also listed as outputs or as an i/o.)
- "r" constraint used instead of "b" for gprs used in insns where the
  r0 encoding means zero rather than r0.
- There were unused asm inputs too.
- All memory was clobbered rather than hooking up memory outputs with
  proper memory constraints, and that and the lack of proper memory
  input constraints meant the asms needed to be volatile and their
  containing function noinline.
- Some parameters were being passed unnecessarily via memory.
- When a copy of a