scripts: Update download locations

This change updates the download locations to default to the official
download site.

For gcc and gdb, also separate out the linaro download locations so that
if you are downloading the linaro variant, it skips trying to download
from the official gcc mirror.

This commit closes #3

Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>
Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>