powerpc/traps: machine_check_generic() is only used on 32-bit

Make it clear that the fallback version of machine_check_generic() is
only used on 32-bit configs.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/traps.c

@@ -618,7 +618,7 @@ int machine_check_8xx(struct pt_regs *regs)
 	return 0;
 #endif
 }
-#else
+#elif defined(CONFIG_PPC32)
 int machine_check_generic(struct pt_regs *regs)
 {
 	unsigned long reason = regs->msr;