runtime: Fix GDB integration with Python 2
A similar fix was applied in
545686857bc4c2e7a5306d97e5ef48f631d277bc
but another instance of 'pc' was missed.
Also adds a test for the goroutine gdb command.
It currently uses goroutine 2 for the test, since goroutine 1 has
its stack pointer set to 0 for some reason.
Change-Id: I53ca22be6952f03a862edbdebd9b5c292e0853ae
Reviewed-on: https://go-review.googlesource.com/8729
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>