unix: convert openbsd/arm64 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: I836a484b14e0a427ac565315e27f0de1e9a5d021
Reviewed-on: https://go-review.googlesource.com/c/sys/+/421796
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

unix/zsysnum_openbsd_arm64.go

@@ -6,6 +6,7 @@
 
 package unix
 
+// Deprecated: Use libc wrappers instead of direct syscalls.
 const (
 	SYS_EXIT           = 1   // { void sys_exit(int rval); }
 	SYS_FORK           = 2   // { int sys_fork(void); }