runtime: initial windows/arm64 implementation files

This CL adds a few small files - defs, os, and rt0 - to start
on windows/arm64 support for the runtime.

It also copies sys_windows_arm.s to sys_windows_arm64.s,
with the addition of "#ifdef NOT_PORTED" around the entire file.
This is meant to make future CLs easier to review, since the
general pattern is to translate the 32-bit ARM assembly into
64-bit ARM assembly.

This CL is part of a stack adding windows/arm64
support (#36439), intended to land in the Go 1.17 cycle.

Change-Id: I922037eb3890e77bac48281ecaa8e489595675be
Reviewed-on: https://go-review.googlesource.com/c/go/+/288827


Trust: Russ Cox <rsc@golang.org>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>