GDB release tarball format change

Hello,

I reviewed again all emails in the following discussion:
https://www.sourceware.org/ml/gdb/2012-01/msg00078.html

I think I can say that there was a general agreement towards
adopting the xz compression format to replace the use of
the bzip2 fomat. To help legacy system support, we will continue
producing .tar.gz tarballs as well.

Unless there are new objections against this change, I am planning
on implementing it for the upcoming GDB 7.8 release.

-- 
Joel