py: Add option to compiler to specify default code emitter.

Also add command line option to unix port to select emitter.
diff --git a/py/builtinevex.c b/py/builtinevex.c
index 0a92819..fc3a601 100644
--- a/py/builtinevex.c
+++ b/py/builtinevex.c
@@ -33,7 +33,7 @@
     }
 
     // compile the string
-    mp_obj_t module_fun = mp_compile(pn, source_name, false);
+    mp_obj_t module_fun = mp_compile(pn, source_name, MP_EMIT_OPT_NONE, false);
     mp_parse_node_free(pn);
 
     if (module_fun == mp_const_none) {