[release-branch.go1.4] runtime: stop using sigreturn on openbsd/386

In future releases of OpenBSD, the sigreturn syscall will no longer
exist. As such, stop using sigreturn on openbsd/386 and just return
from the signal trampoline.

This is a backport of https://golang.org/cl/23024 to the Go 1.4 branch.

Updates #20672

Change-Id: Ia1caf9427c51a6871f51dbff06720f3158e38c68
Reviewed-on: https://go-review.googlesource.com/66071


Reviewed-by: Ian Lance Taylor <iant@golang.org>