undo CL 7301062 / 9742f722b558

broke arm garbage collector

traceback_arm fails with a missing pc. It needs CL 7494043.
But that only makes the build break later, this time with
"invalid freelist". Roll back until it can be fixed correctly.

??? original CL description
runtime: restrict stack root scan to locals and arguments

R=rsc
CC=golang-dev
https://codereview.appspot.com/7301062
???

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7493044