[release-branch.go1.4] runtime, syscall: update openbsd for changes to syskill

Change the openbsd runtime to use the current sys_kill and sys_thrkill
system calls.

This is a backport of https://golang.org/cl/34093 to the Go 1.4 branch.

Updates #20672

Change-Id: I4fde5bad2fbfbe99196053d69ba235b49ddb32c8
Reviewed-on: https://go-review.googlesource.com/66072


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