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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 (as we already do for openbsd/amd64 and
openbsd/arm).

Change-Id: Ic4de1795bbfbfb062a685832aea0d597988c6985
Reviewed-on: https://go-review.googlesource.com/23024

Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>