Add Makefile for yasm on Windows.

Add Makefile for yasm on Windows.

Makefile for yasm is missing on Windows, causing build to fail.
Building YASM on Windows is required to build libvpx and ffmpeg.

BUG=None
TEST=None

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=90800

[Wez, 2011-06-27 20:31:01]

Where does has this Makefile come from?  Would it make sense to gyp-ify it
instead of just checking it in?

[Alpha Left Google, 2011-06-27 20:32:30]

On 2011/06/27 20:31:01, Wez wrote:
> Where does has this Makefile come from?  Would it make sense to gyp-ify it
> instead of just checking it in?

This is coming from the linux version with slight modifications. We can't use
gyp to replace this because YASM use this as an input to generate a .c file
instead of using it as a build rule.

[Wez, 2011-06-27 21:27:07]

On 2011/06/27 20:32:30, Alpha wrote:
> On 2011/06/27 20:31:01, Wez wrote:
> > Where does has this Makefile come from?  Would it make sense to gyp-ify it
> > instead of just checking it in?
> 
> This is coming from the linux version with slight modifications. We can't use
> gyp to replace this because YASM use this as an input to generate a .c file
> instead of using it as a build rule.

LGTM.