syscall: point to x/sys in DLL loading docs, update syscall generator

Updates the syscall generator for patchset 4 of https://golang.org/cl/21388.

Updates #14959

Change-Id: Icbd6df489887d3dcc076dfc73d4feb1376abaf8b
Reviewed-on: https://go-review.googlesource.com/21428


Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-on: https://go-review.googlesource.com/21697


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>