syscall: call getfsstat via libc on openbsd

syscall: call getfsstat via libc on openbsd

On openbsd, call getfsstat directly via libc, instead of calling it
via syscall.Syscall.

Updates #63900

Change-Id: Ib4c581160b170e6cc6017c42e959e647d97ac993
Reviewed-on: https://go-review.googlesource.com/c/go/+/538736
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Josh Rickmar <jrick@zettaport.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>