py: Include mpconfig.h before all other includes.

It defines types used by all other headers.

Fixes #691.
diff --git a/py/asmx64.c b/py/asmx64.c
index 6c22ea2..4695bdc 100644
--- a/py/asmx64.c
+++ b/py/asmx64.c
@@ -29,8 +29,8 @@
 #include <assert.h>
 #include <string.h>
 
-#include "misc.h"
 #include "mpconfig.h"
+#include "misc.h"
 
 // wrapper around everything in this file
 #if MICROPY_EMIT_X64