powerpc: machine_check_generic is wrong on 64bit

Decoding machine checks is CPU specific and so machine_check_generic doesn't
do the right thing on 64bit chips. Luckily we never call into this code
because we call ppc_md.machine_check_exception instead if available.

Since we check cur_cpu_spec->machine_check before calling it, we may as
well remove machine_check_generic from 64bit archs.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>