runtime,syscall: convert syscall on openbsd/arm64 to libc

Convert the syscall package on openbsd/arm64 to use libc rather than performing
direct system calls.

Updates #36435

Change-Id: I7e1da8537cea9ed9bf2676f181e56ae99383333f
Reviewed-on: https://go-review.googlesource.com/c/go/+/286815


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>