syscall: replace mksyscall_windows.go with wrapper to new x/sys home

We replace the existing file with a thin wrapper around its target so
that we don't break anybody's workflow.

Updates #34388

Change-Id: I0d00371c483cb78f4be18fe987df33c79cd40f05
Reviewed-on: https://go-review.googlesource.com/c/go/+/198977
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>