RFR: 8007446: Add /MP to cl.exe speeds up windows builds of OpenJDK.

[Erik Joelsson]

Please review this simple patch adding /MP to the windows cl.exe command 
lines when supported. In my tests, the speed improvement is around 40% 
on the hotspot part of the build on the Stockholm jprt queue and my 
local build. This is really low hanging fruit for build speed improvements.

http://cr.openjdk.java.net/~erikj/8007446/webrev.hotspot.01/
I will also need a sponsor to push this for me if review is passed.

/Erik