Add openbsd 386 syscall support, partially based on the existing
freebsd 386 syscall implementation.

FTR zerrors_openbsd_386.go cannot currently be completely built on
openbsd/i386 due to what appears to be a gcc bug. The constants can be
successfully generated with -m32 on openbsd/amd64 and the error
table can then be generated on openbsd/i386.

R=rsc
CC=golang-dev
https://golang.org/cl/4969045