syscall: fix mkall.sh for openbsd/386

syscall: fix mkall.sh for openbsd/386

CL 287654 converted the syscall package on openbsd/386 to use libc.
However, the mksyscall.pl invocation wasn't adjusted. Do so now to use
syscall_openbsd_libc.go like the other libc-based openbsd ports.

Change-Id: I48a7bd6ce4c25eca5222f560ed584e412b466111
Reviewed-on: https://go-review.googlesource.com/c/go/+/361481
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>