runtime: factor common code out of defs_windows_*.go

Also give up on the fiction that these files can be regenerated.
They contain many manual edits, and they're fairly small anyway.

This CL is part of a stack adding windows/arm64
support (#36439), intended to land in the Go 1.17 cycle.
This CL is, however, not windows/arm64-specific.
It is cleanup meant to make the port (and future ports) easier.

Change-Id: Ib4e4e20a43d8beb1d5390fd184160c33607641f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/288807


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