gcc: arc: Download from github instead of Synopsys website

The github release mechanism only supports .tar.gz, hence the ugly
GCC_UNPACK variable.

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