unix: flip openbsd libc build tags

Rather than listing OpenBSD architectures that are using libc, list the one
that is still blocked on direct system calls. This will simplify the addition
of new OpenBSD ports.

Updates golang/go#36435

Change-Id: I8d1d16a033245af436d955b5fa6ceeaa1f2a9f6b
Reviewed-on: https://go-review.googlesource.com/c/sys/+/439975
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_openbsd_libc.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build (openbsd && 386) || (openbsd && amd64) || (openbsd && arm) || (openbsd && arm64)
-// +build openbsd,386 openbsd,amd64 openbsd,arm openbsd,arm64
+//go:build openbsd && !mips64
+// +build openbsd,!mips64
 
 package unix