[foreign-abi] RFR 8228509: Add support for binding varargs functions on Windows

[Jorn Vernee]

Hi,

Please review the following patch which adds support for using varargs 
functions with float arguments on Windows (e.g. printf).

Bug: https://bugs.openjdk.java.net/browse/JDK-8228509
Webrev: 
http://cr.openjdk.java.net/~jvernee/panama/webrevs/8228509/webrev.00/

This mainly consists of a way to mark varargs argument layouts as such 
through the use of an annotation, which is then detected when computing 
a CallingSequence.

I've also added a general purpose test for testing the passing of vararg 
arguments.

Thanks,
Jorn