The -systemdll and -xsys flags generate broken code in some situations
(see issue for details). Fix all that.

This CL only fixes bugs in existing code, but I have more changes comming:

golang.org/x/sys/windows is not the only package that uses mksyscall_windows.go.
golang.org/x/exp/shiny and github.com/derekparker/delve do too. I also have
few personal packages that use mksyscall_windows.go. None of those packages
are aware of new -xsys flag. I would like to change mksyscall_windows.go, so
external packages do not need to use -xsys flag. I would love to get rid of
-xsys flag altogether, but I don't see how it is possible. So I will, probably,
replace -xsys with a flag that means opposite to -xsys, and use new flag
everywhere in standard libraries. Flag name suggestions are welcome.

-systemdll flag makes users code more "secure". I would like to make -systemdll
behaviour a default for all mksyscall_windows.go users. We use that already in
standard library. If we think "secure" is important, we should encourage it in
all users code. If mksyscall_windows.go user insist on using old code, provide
-use_old_loaddll (need good name here) flag for that. So -systemdll flag will
be replaced with -use_old_loaddll.

Fixes #15167

Change-Id: I516369507867358ba1b66aabe00a17a7b477016e
Reviewed-on: https://go-review.googlesource.com/21645Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>