This leads to ~30kB improvement on code size for ARM machines with VFP/NEON.
Example: go test -c math
       GOARM=5  GOARM=6
  Old: 1884200  1839144
  New: 1884165  1805245
  -:        35    33899

R=rsc, bradfitz, dave, kai.backman
CC=golang-dev
https://golang.org/cl/5975060