runtime: adjust traceback / garbage collector boundary
The garbage collection routine addframeroots is duplicating
logic in the traceback routine that calls it, sometimes correctly,
sometimes incorrectly, sometimes incompletely.
Pass necessary information to addframeroots instead of
deriving it anew.
Should make addframeroots significantly more robust.
It's certainly smaller.
Also try to standardize on uintptr for saved pc, sp values.
Will make CL
10036044 trivial.
R=golang-dev, dave, dvyukov
CC=golang-dev
https://golang.org/cl/
10169045