syscall, internal/syscall/windows, internal/syscall/windows/registry: make go generate work on every os

Fixes #16368

Change-Id: I2ef7a2deb5798e11cc1d3f8ca29a6e1655155422
Reviewed-on: https://go-review.googlesource.com/27411Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>