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

Also add command line option to unix port to select emitter.

py/compile.h

@@ -1 +1,9 @@
-mp_obj_t mp_compile(mp_parse_node_t pn, qstr source_file, bool is_repl);
+enum {
+    MP_EMIT_OPT_NONE,
+    MP_EMIT_OPT_BYTE_CODE,
+    MP_EMIT_OPT_NATIVE_PYTHON,
+    MP_EMIT_OPT_VIPER,
+    MP_EMIT_OPT_ASM_THUMB,
+};
+
+mp_obj_t mp_compile(mp_parse_node_t pn, qstr source_file, uint emit_opt, bool is_repl);