gdb: add version 7.9

Add gdb version 7.9, and adjust the tarball logic to default to .tar.xz
(for 7.8 & 7.9), fall back to .tar.bz2 for 7.7.x and leave the rest
alone.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>