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>

include/asm-powerpc/machdep.h

@@ -253,7 +253,11 @@ extern struct machdep_calls *machine_id;
 
 #define __machine_desc __attribute__ ((__section__ (".machine.desc")))
 
-#define define_machine(name) struct machdep_calls mach_##name __machine_desc =
+#define define_machine(name)					\
+	extern struct machdep_calls mach_##name;		\
+	EXPORT_SYMBOL(mach_##name);				\
+	struct machdep_calls mach_##name __machine_desc =
+
 #define machine_is(name) \
 	({ \
 		extern struct machdep_calls mach_##name \