"info goroutines" is failing because it hasn't kept up with changes in
the 1.5 runtime.  This fixes three issues preventing "info goroutines"
from working.  allg is no longer a linked list, so switch to using the
allgs slice.  The g struct's 'status' field is now called
'atomicstatus', so rename uses of 'status'.  Finally, this was trying
to parse str(pc) as an int, but str(pc) can return symbolic
information after the raw hex value; fix this by stripping everything
after the first space.

This also adds a test for "info goroutines" to runtime-gdb_test, which
was previously quite skeletal.

Change-Id: I8ad83ee8640891cdd88ecd28dad31ed9b5833b7a
Reviewed-on: https://go-review.googlesource.com/4935Reviewed-by: Minux Ma <minux@golang.org>