gcc: fix location of snapshots

The gcc snapshots are now located at
ftp://gcc.gnu.org/pub/gcc/snapshots/

. This has been tested with a
recent 4.8.0-RC snapshot.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>