syscall: switch go:generate directives back to mksyscall_windows.go

Adjust mksyscall_windows.go to activate module mode and set
-mod=readonly, and to suppress its own deprecation warning when run
from within GOROOT/src.

We can't vendor the mkwinsyscall tool in to the std module directly,
because std-vendored dependencies (unlike the dependencies of all
other modules) turn into actual, distinct packages in 'std' when
viewed from outside the 'std' module. We don't want to introduce a
binary in the 'std' meta-pattern, but we also don't particularly want
to add more special-cases to the 'go' command right now when we have
an existing wrapper program that can do the job.

I also regenerated the affected packages to ensure that they are
consistent with the current version of mksyscall, which produced some
declaration-order changes in
internal/syscall/windows/zsyscall_windows.go.

Fixes #41916
Updates #25922

Change-Id: If6e6f8ba3dd372a7ecd6820ee6c0ca38d55f0f35
Reviewed-on: https://go-review.googlesource.com/c/go/+/261499


Trust: Bryan C. Mills <bcmills@google.com>
Trust: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>