runtime: mark sigtramp as TOPFRAME on the rest of unix

runtime: mark sigtramp as TOPFRAME on the rest of unix

This extends CL 402190 from Linux to the rest of the Unix OSes.

Marking sigtramp as TOPFRAME allows gentraceback to stop tracebacks at
the end of a signal handler, since there is not much beyond sigtramp.

Change-Id: I8b7f5d55d41889f59c0a79c65351b9b0b2d77717
Reviewed-on: https://go-review.googlesource.com/c/go/+/402934
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Austin Clements <austin@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>