x/debug: read goroutines from slice runtime.allgs

Tries to read runtime.{allg,allglen} if we can't read runtime.allgs.
runtime.allg is not present in Go 1.6 executables.

Change-Id: I68e08fa4cfb1859e083e91beb8b25db417c2a057
Reviewed-on: https://go-review.googlesource.com/19940
Reviewed-by: Dave Day <djd@golang.org>
3 files changed