cloog: add mirror

The bastoul URL has been flagged (inappropriately, I think) as a botnet
by Sonic Wall, a large firewall and content filter provider. Adding
a mirror will make it easier for people behind corporate SonicWall
filters to access this GCC dependency.

Fixes #27937.
Closes #27952.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>