The previous syscall constants regeneration on openbsd was conducted
with OpenBSD current 3 months ago and it missed updating openbsd/386.
This CL adds TIOCGSID for fixing the inconsistency between opensbd/amd64
and openbsd/386.

Update #7049

LGTM=iant
R=jsing, rsc, iant
CC=golang-codereviews
https://golang.org/cl/96960044