[nicl] Adding --exclude-symbols option to jextract tool

[Sundararajan Athijegannathan]

Please review 
http://cr.openjdk.java.net/~sundar/jextract_exclude_symbols/webrev.00/

By default jextract generates interface that includes all the native 
declarations in the input header files. --exclude-symbol option can be 
used to filter native symbols in the input header files.

Thanks,
-Sundar