When we receive a signal, if G is nil we call badsignal, which
calls needm. When cgo is not used, there is no extra M, so needm
will just hang. In this situation, even GOTRACEBACK=crash cannot
get a stack trace, as we're in the signal handler and cannot
receive another signal (SIGQUIT).

Instead, just crash.

For #35554.
Updates #34391.

Change-Id: I061ac43fc0ac480435c050083096d126b149d21f
Reviewed-on: https://go-review.googlesource.com/c/go/+/206959
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>