package/gcc: cleanup arch/cpu combinations

Cleanup arch/cpu combination limits, we had super-wide depends and it
doesn't help readability, version bumps or testing.

Make the bool/depends/select order the same for all entries.

Drop redundant limitations, for example sparc* if sparc wasn't
supported in general.

Power8 requires at least gcc 4.9.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>