A similar fix was applied in 54568685
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>