runtime: if traceback sees a breakpoint, don't change the PC
runtime: if traceback sees a breakpoint, don't change the PC

Changing the PC confuses gdb, because execution does not
continue where gdb expects it.  Not changing the PC has the
potential to confuse a stack dump, but when running under gdb
it seems better to confuse a stack dump than to confuse gdb.

Fixes issue 6776.

LGTM=rsc
R=golang-codereviews, dvyukov, rsc
CC=golang-codereviews
https://codereview.appspot.com/49580044

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