gdb: bump to 7.8.1

This commit bumps to 7.8.1, and updates the logic to use .tar.xz
tarballs so that it applies to all 7.8.x versions.

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