buildroot

gdb: specify hashes for the ARC and Microblaze versions

On ARC and Microblaze, a special GDB version is used, fetched from
Github in both cases. Now that we consider Github generated tarballs
to be stable, we can specify hashes for those versions, rather than
using the special "none" hash to skip the hash check.

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