runtime: do not treat asmcgocall as a topofstack on g0

This was added in 2018 to fix a runtime crash during unwind
during a unhandled-panic-induced crash.
(See https://golang.org/cl/90895 and #23576.)
Clearly we cannot unwind past this function, and the change
did stop the unwind. But it's not a top-of-stack function, and
the real issue is that SP is changed.

The new SPWRITE bit takes care of this instead, so we can drop
it from the topofstack function.

At this point the topofstack function is only checking the
TOPFRAME bit, so we can inline that into the one call site.

This CL is part of a stack adding windows/arm64
support (#36439), intended to land in the Go 1.17 cycle.
This CL is, however, not windows/arm64-specific.
It is cleanup meant to make the port (and future ports) easier.

Change-Id: I856552298032770e48e06c95a20823a1dbd5e38c
Reviewed-on: https://go-review.googlesource.com/c/go/+/288805


Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>