[foreign-jextract] RFR Add varargs support for jextract generated code

[sundararajan.athijegannathan at oracle.com]

Please review.

Bug: https://bugs.openjdk.java.net/browse/JDK-8237646

Webrev: 
http://cr.openjdk.java.net/~sundar/minimal_jextract_varargs/webrev.00/

PS. Piggybacking to add ToolProvider for jextract tool and basic test 
framework with simple generation tests.

Thanks,

-Sundar