runtime: record full frame size for arm _sfloat2
runtime: record full frame size for arm _sfloat2

With preemption, _sfloat2 can show up in stack traces.
Write the function prototype in a way that accurately
shows the frame size and the fact that it might contain
pointers.

R=golang-dev, dvyukov
CC=golang-dev
https://codereview.appspot.com/11523043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang