Allow ports to define statically builtin functions.

Convert unix open() to such.
diff --git a/unix/mpconfigport.h b/unix/mpconfigport.h
index dc92970..5b2503f 100644
--- a/unix/mpconfigport.h
+++ b/unix/mpconfigport.h
@@ -36,3 +36,8 @@
 typedef double machine_float_t;
 
 machine_float_t machine_sqrt(machine_float_t x);
+
+struct _mp_obj_fun_native_t;
+extern const struct _mp_obj_fun_native_t mp_builtin_open_obj;
+#define MICROPY_EXTRA_BUILTINS \
+    { MP_QSTR_open, (mp_obj_t)&mp_builtin_open_obj },