py: Add global default_emit_opt variable to make emit kind persistent.

mp_compile no longer takes an emit_opt argument, rather this setting is now
provided by the global default_emit_opt variable.

Now, when -X emit=native is passed as a command-line option, the emitter
will be set for all compiled modules (included imports), not just the
top-level script.

In the future there could be a way to also set this variable from a script.

Fixes issue #4267.

lib/utils/pyexec.c

@@ -89,7 +89,7 @@ STATIC int parse_compile_execute(const void *source, mp_parse_input_kind_t input
             // source is a lexer, parse and compile the script
             qstr source_name = lex->source_name;
             mp_parse_tree_t parse_tree = mp_parse(lex, input_kind);
-            module_fun = mp_compile(&parse_tree, source_name, MP_EMIT_OPT_NONE, exec_flags & EXEC_FLAG_IS_REPL);
+            module_fun = mp_compile(&parse_tree, source_name, exec_flags & EXEC_FLAG_IS_REPL);
             #else
             mp_raise_msg(&mp_type_RuntimeError, "script compilation not supported");
             #endif