runtime: clean up windows a bit

Document the various hard-coded architecture checks
or remove them in favor of more general checks.
This should be a no-op now but will make the arm64 port
have fewer diffs.

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: Ifd6b19e44e8c9ca4a0d2590f314928ce235821b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/288813


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