arc: Add option for ARC-specific download site

As ARC support is not yet in all upstream packages, a different location is
required to download the packages from. This adds an option to specify a
site for ARC-specific versions of packages such as binutils, gcc.

When ARC support has been upstreamed for all packages, this option can be
removed again.

Signed-off-by: Mischa Jonker <mjonker@synopsys.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>