all: drop support for Windows Vista or below (Windows XP)

Per the notice in the Go 1.10 release notes, this change drops the
support for Windows Vista or below (including Windows XP) and
simplifies the code for the sake of maintenance.

There is one exception to the above. The code related to DLL and
system calls still remains in the runtime package. The remaining code
will be refined and used for supporting upcoming Windows versions in
future.

Updates #17245
Fixes #23072

Change-Id: I9e2821721f25ef9b83dfbf85be2b7ee5d9023aa5
Reviewed-on: https://go-review.googlesource.com/94255


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>