runtime: mark OpenBSD raise function nosplit

It is called by the signal handler before switching to gsignal
(sigtrampgo -> sigfwdgo -> dieFromSignal -> raise)
which means that it must not split the stack.

All other instances of raise are already marked nosplit.

Fixes #40076

Change-Id: I4794491331af48c46d0d8ebc82d34c6483f0e6cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/241121


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>