Based on the calling convention documented in:

	https://msdn.microsoft.com/en-us/library/zthk2dkh.aspx

and long-used in golang.org/x/mobile/gl via some fixup asm:

	https://go.googlesource.com/mobile/+/master/gl/work_windows_amd64.s

Fixes #6510

Change-Id: I97e81baaa2872bcd732b1308915eb66f1ba2168f
Reviewed-on: https://go-review.googlesource.com/32173
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Minux Ma <minux@golang.org>