runtime: use runtime·callers when racefuncenter's pc is on the heap.
runtime: use runtime·callers when racefuncenter's pc is on the heap.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6821069

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