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 <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

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