powerpc/32: Generate miboot images with ARCH=powerpc

Miboot images are apparently still used on some old 32-bit powermacs,
so build them with ARCH=powerpc if we're 32-bit and powermac support
is enabled.

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

arch/powerpc/boot/dummy.c

@@ -0,0 +1,4 @@
+int main(void)
+{
+	return 0;
+}