py: Add support to save native, viper and asm code to .mpy files.

This commit adds support for saving and loading .mpy files that contain
native code (native, viper and inline-asm).  A lot of the ground work was
already done for this in the form of removing pointers from generated
native code.  The changes here are mainly to link in qstr values to the
native code, and change the format of .mpy files to contain native code
blocks (possibly mixed with bytecode).

A top-level summary:

- @micropython.native, @micropython.viper and @micropython.asm_thumb/
  asm_xtensa are now allowed in .py files when compiling to .mpy, and they
  work transparently to the user.

- Entire .py files can be compiled to native via mpy-cross -X emit=native
  and for the most part the generated .mpy files should work the same as
  their bytecode version.

- The .mpy file format is changed to 1) specify in the header if the file
  contains native code and if so the architecture (eg x86, ARMV7M, Xtensa);
  2) for each function block the kind of code is specified (bytecode,
  native, viper, asm).

- When native code is loaded from a .mpy file the native code must be
  modified (in place) to link qstr values in, just like bytecode (see
  py/persistentcode.c:arch_link_qstr() function).

In addition, this now defines a public, native ABI for dynamically loadable
native code generated by other languages, like C.

py/emitglue.h

@@ -48,6 +48,11 @@ typedef enum {
     MP_CODE_NATIVE_ASM,
 } mp_raw_code_kind_t;
 
+typedef struct _mp_qstr_link_entry_t {
+    uint16_t off;
+    uint16_t qst;
+} mp_qstr_link_entry_t;
+
 typedef struct _mp_raw_code_t {
     mp_uint_t kind : 3; // of type mp_raw_code_kind_t
     mp_uint_t scope_flags : 7;
@@ -58,6 +63,11 @@ typedef struct _mp_raw_code_t {
     size_t fun_data_len;
     uint16_t n_obj;
     uint16_t n_raw_code;
+    #if MICROPY_EMIT_NATIVE || MICROPY_EMIT_INLINE_ASM
+    uint16_t prelude_offset;
+    uint16_t n_qstr;
+    mp_qstr_link_entry_t *qstr_link;
+    #endif
     #endif
     #if MICROPY_EMIT_NATIVE || MICROPY_EMIT_INLINE_ASM
     mp_uint_t type_sig; // for viper, compressed as 2-bit types; ret is MSB, then arg0, arg1, etc
@@ -75,7 +85,15 @@ void mp_emit_glue_assign_bytecode(mp_raw_code_t *rc, const byte *code,
     uint16_t n_obj, uint16_t n_raw_code,
     #endif
     mp_uint_t scope_flags);
-void mp_emit_glue_assign_native(mp_raw_code_t *rc, mp_raw_code_kind_t kind, void *fun_data, mp_uint_t fun_len, const mp_uint_t *const_table, mp_uint_t n_pos_args, mp_uint_t scope_flags, mp_uint_t type_sig);
+
+void mp_emit_glue_assign_native(mp_raw_code_t *rc, mp_raw_code_kind_t kind, void *fun_data, mp_uint_t fun_len,
+    const mp_uint_t *const_table,
+    #if MICROPY_PERSISTENT_CODE_SAVE
+    uint16_t prelude_offset,
+    uint16_t n_obj, uint16_t n_raw_code,
+    uint16_t n_qstr, mp_qstr_link_entry_t *qstr_link,
+    #endif
+    mp_uint_t n_pos_args, mp_uint_t scope_flags, mp_uint_t type_sig);
 
 mp_obj_t mp_make_function_from_raw_code(const mp_raw_code_t *rc, mp_obj_t def_args, mp_obj_t def_kw_args);
 mp_obj_t mp_make_closure_from_raw_code(const mp_raw_code_t *rc, mp_uint_t n_closed_over, const mp_obj_t *args);