JFFI needs binaries for more arches

The current release of jffi bundled with JRuby has binaries for Darwin (all arches), i386-Linux, i386-Windows, i386-SunOS and x86_64-SunOS. It would be beneficial to have binaries for at least x86_64-Linux, and maybe more esoteric platforms like i386-FreeBSD and x86_64-FreeBSD.

To build jffi, do something like:

 hg clone -r0.6 https://kenai.com/hg/jffi~core  jffi-0.6
 cd jffi-0.6
 ant jar test
 cp dist/x86_64-Linux.jar /path/to/jruby/build_lib/jffi-x86_64-Linux.jar
 cd /path/to/jruby
 ant clean test

The resulting dist/$CPU-$OS.jar file could also be attached to this issue, and someone could commit it to jruby.