runtime: use TOPFRAME to identify top-of-frame functions

No change to actual runtime, but helps reduce the laundry list
of functions.

mcall, morestack, and asmcgocall are not actually top-of-frame,
so those need more attention in follow-up CLs.

mstart moved to assembly so that it can be marked TOPFRAME.

Since TOPFRAME also tells DWARF consumers not to unwind
this way, this change should also improve debuggers a
marginal amount.

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: If1e0d46ca973de5e46b62948d076f675f285b5d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/288802


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