runtime/gdb: use goroutine atomicstatus to determine the state

Previously find_goroutine determined whether a goroutine is
stopped by checking the sched.sp field. This heuristic doesn't
always hold but causes find_goroutine to return bogus pc/sp
info for running goroutines.

This change uses the atomicstatus bit to determine
the state which is more accurate.

R=go1.11

Change-Id: I537d432d9e0363257120a196ce2ba52da2970f59
Reviewed-on: https://go-review.googlesource.com/49691


Reviewed-by: Austin Clements <austin@google.com>