Power8 inline assembly fixes

[martin-frbg]

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 pointer input parm was needed, the value passed to
  the asm was incremented in C and decremented in asm, rather than
  using i/o parms, an early clobber constraint, or a temp output reg
  copied in the asm. In most cases a small change to assembly could
  be made that obviated the need for the extra pointer.
• A number of functions did not compute the final sum or dot-product
  in assembly, instead using scalar code in C.
• dcbt was bogus.