re-instate linux 64bit dist build

This reverts part of a commit mbolivar made a long time ago ("QA testing improvements; better snapshot file names.") which removed Linux 64bit IDE builds.

The in-line comment said "FIXME re-insert this once we've actually got a 64 bit version". I believe the x86_64 .jar files work fine on 64bit linux distros, while the default 32bit ones do not.