Make the @micropython.native decorator no-op if support isn't enabled

When adding the ability for boards to turn on the `@micropython.native`, `viper`, and `asm_thumb` decorators it was pointed out that it's somewhat awkward to write libraries and drivers that can take advantage of this since the decorators raise `SyntaxErrors` if they aren't enabled. In the case of `viper` and `asm_thumb` this behavior makes sense as they require writing non-normative code. Drivers could have a normal and viper/thumb implementation and implement them as such:

```python
try:
    import _viper_impl as _impl
except SyntaxError:
    import _python_impl as _impl

def do_thing():
    return _impl.do_thing()
```

For `native`, however, this behavior and the pattern to work around it is less than ideal. Since `native` code should also be valid Python code (although not necessarily the other way around) using the pattern above means *duplicating* the Python implementation and adding `@micropython.native` in the code. This is an unnecessary maintenance burden.

This commit *modifies* the behavior of the `@micropython.native` decorator. On boards with `CIRCUITPY_ENABLE_MPY_NATIVE` turned on it operates as usual. On boards with it turned off it does *nothing*- it doesn't raise a `SyntaxError` and doesn't apply optimizations. This means we can write our drivers/libraries once and take advantage of speedups on boards where they are enabled.

py/compile.c

@@ -775,13 +775,22 @@ STATIC bool compile_built_in_decorator(compiler_t *comp, int name_len, mp_parse_
     qstr attr = MP_PARSE_NODE_LEAF_ARG(name_nodes[1]);
     if (attr == MP_QSTR_bytecode) {
         *emit_options = MP_EMIT_OPT_BYTECODE;
-#if MICROPY_EMIT_NATIVE
+    // @micropython.native decorator.
     } else if (attr == MP_QSTR_native) {
+        // Different from MicroPython: native doesn't raise SyntaxError if native support isn't
+        // compiled, it just passes through the function unmodified.
+        #if MICROPY_EMIT_NATIVE
         *emit_options = MP_EMIT_OPT_NATIVE_PYTHON;
+        #else
+        return true;
+        #endif
+    #if MICROPY_EMIT_NATIVE
+    // @micropython.viper decorator.
     } else if (attr == MP_QSTR_viper) {
         *emit_options = MP_EMIT_OPT_VIPER;
-#endif
+    #endif
     #if MICROPY_EMIT_INLINE_ASM
+    // @micropython.asm_thumb decorator.
     } else if (attr == ASM_DECORATOR_QSTR) {
         *emit_options = MP_EMIT_OPT_ASM;
     #endif