Since CL 10087 build tags for GOOS and GOARCH are emitted by
mksysctl_openbsd.go but the generated file is always written to
zsysctl_openbsd.go.

Adjust mkall.sh to write to GOOS/GOARCH specific files and create a
zsysctl_openbsd_${GOARCH}.go file for each GOARCH supported on openbsd.

Change-Id: Id2a7b2815081f024de0ca87ba870a672bab32d24
Reviewed-on: https://go-review.googlesource.com/68870
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>