unix: convert openbsd/amd64 to direct libc calls

The current code has continued to work on OpenBSD, since it has been using
syscall(2) via libc. However, the system call numbers are still hardcoded in
golang.org/sys/unix. Various system call changes have been made in OpenBSD,
resulting in changes to the system call numbers and arguments, which now
fail when this package is used.

Switch to calling various system calls directly via libc, rather than calling
via libc using syscall(2).

Updates golang/go#36435

Change-Id: Iadd5734fd4a2421d758883293bd8a6759fc52ca7
Reviewed-on: https://go-review.googlesource.com/c/sys/+/421797
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@google.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 && arm64
-// +build openbsd,arm64
+//go:build (openbsd && amd64) || (openbsd && arm64)
+// +build openbsd,amd64 openbsd,arm64
 
 package unix