gdb: add version 7.8

This commit adds support for the 7.8 version of gdb. Note that the
tarball of this version is not available as a .tar.bz2, so we have to
add a special case and download the .tar.xz for this version.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>