make.rules: Compile SLOF for power5

By default, SLOF would implement the cpu_to_le64() helper with
the 'stdbrx' instruction which is invalid under POWER5+ and 970
CPUs. This breaks the QEMU pseries machine with such CPUs when
virtio or USB devices or in use.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>