Add `mkdir build` in makefile.

It this is missing, building with a clean repository will fail
because build/ does not exist.

kernel/Makefile

@@ -143,6 +143,7 @@ $(kernel): kernel $(assembly_object_files) $(linker_script)
 		$(assembly_object_files) target/x86_64-blog_os/$(mode)/libucore.a
 
 kernel:
+	@mkdir build/$(arch) -p
 	@CC=$(cc) cargo xbuild $(build_args)
 
 # compile assembly files