[release-branch.go1.2] runtime: if traceback sees a breakpoint, don't change the PC

««« CL 49580044 / 38cd458b1dfe
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 #6776.

LGTM=rsc
R=golang-codereviews, dvyukov, rsc
CC=golang-codereviews
https://golang.org/cl/49580044
»»»

LGTM=r
R=golang-codereviews, r
CC=golang-dev
https://golang.org/cl/69800043