powerpc: Fix define_machine so machine_is() works from modules

machine_is() was always returning 0 when used in a module, because
we weren't exporting the machine definitions.  This was why sound
wasn't working on powermacs when CONFIG_SND_POWERMAC=m.  Original
fix from Ben Herrenschmidt, further fixed by me.

Signed-off-by: Paul Mackerras <paulus@samba.org>