Currently isSystemGoroutine has a hard-coded list of known entry
points into system goroutines. This list is annoying to maintain. For
example, it's missing the ensureSigM goroutine.

Replace it with a check that simply looks for any goroutine with
runtime function as its entry point, with a few exceptions. This also
matches the definition recently added to the trace viewer (CL 81315).

Change-Id: Iaed723d4a6e8c2ffb7c0c48fbac1688b00b30f01
Reviewed-on: https://go-review.googlesource.com/81655
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>