runtime: mark sigtramp as TOPFRAME

runtime: mark sigtramp as TOPFRAME

Currently throw() in the signal handler results in "fatal error: unknown
return pc from runtime.sigreturn ...".

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

This is just done on Linux for now, but may apply to other Unix systems
as well.

Change-Id: I96edcb945283f417a5bfe00ce2fb2b1a0d578692
Reviewed-on: https://go-review.googlesource.com/c/go/+/402190
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>