syscall: use go generate to build zsyscall_windows.go

I would like to create new syscalls in src/internal/syscall,
and I prefer not to add new shell scripts for that.

Replacement for CL 136000043.

Change-Id: I840116b5914a2324f516cdb8603c78973d28aeb4
Reviewed-on: https://go-review.googlesource.com/1940


Reviewed-by: Russ Cox <rsc@golang.org>