x86: unify CPU_TYPE for legacy and geode

According to some reports, -march=pentium-mmx is a better choice for
older Geode CPUs than -march=geode anyway.

Bump the minimum architecture of the legacy target from i486 to
pentium-mmx. Anything older is not worth supporting anyway.

Signed-off-by: Felix Fietkau <nbd@nbd.name>