powerpc/64: Make COMPAT user-selectable disabled on littleendian by default.

On bigendian ppc64 it is common to have 32bit legacy binaries but much
less so on littleendian.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/41393d6e895b0d3a47ee62f8f51e1cf888ad6226.1584699455.git.msuchanek@suse.de