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/emitnative.c

@@ -214,6 +214,11 @@ struct _emit_t {
     uint16_t const_table_cur_raw_code;
     mp_uint_t *const_table;
 
+    #if MICROPY_PERSISTENT_CODE_SAVE
+    uint16_t qstr_link_cur;
+    mp_qstr_link_entry_t *qstr_link;
+    #endif
+
     bool last_emit_was_return_value;
 
     scope_t *scope;
@@ -225,6 +230,7 @@ STATIC const uint8_t reg_local_table[REG_LOCAL_NUM] = {REG_LOCAL_1, REG_LOCAL_2,
 
 STATIC void emit_native_global_exc_entry(emit_t *emit);
 STATIC void emit_native_global_exc_exit(emit_t *emit);
+STATIC void emit_native_load_const_obj(emit_t *emit, mp_obj_t obj);
 
 emit_t *EXPORT_FUN(new)(mp_obj_t *error_slot, uint *label_slot, mp_uint_t max_num_labels) {
     emit_t *emit = m_new0(emit_t, 1);
@@ -280,11 +286,29 @@ STATIC void emit_native_mov_reg_state_addr(emit_t *emit, int reg_dest, int local
 }
 
 STATIC void emit_native_mov_reg_qstr(emit_t *emit, int arg_reg, qstr qst) {
+    #if MICROPY_PERSISTENT_CODE_SAVE
+    size_t loc = ASM_MOV_REG_IMM_FIX_U16(emit->as, arg_reg, qst);
+    size_t link_idx = emit->qstr_link_cur++;
+    if (emit->pass == MP_PASS_EMIT) {
+        emit->qstr_link[link_idx].off = loc << 2 | 1;
+        emit->qstr_link[link_idx].qst = qst;
+    }
+    #else
     ASM_MOV_REG_IMM(emit->as, arg_reg, qst);
+    #endif
 }
 
 STATIC void emit_native_mov_reg_qstr_obj(emit_t *emit, int reg_dest, qstr qst) {
+    #if MICROPY_PERSISTENT_CODE_SAVE
+    size_t loc = ASM_MOV_REG_IMM_FIX_WORD(emit->as, reg_dest, (mp_uint_t)MP_OBJ_NEW_QSTR(qst));
+    size_t link_idx = emit->qstr_link_cur++;
+    if (emit->pass == MP_PASS_EMIT) {
+        emit->qstr_link[link_idx].off = loc << 2 | 2;
+        emit->qstr_link[link_idx].qst = qst;
+    }
+    #else
     ASM_MOV_REG_IMM(emit->as, reg_dest, (mp_uint_t)MP_OBJ_NEW_QSTR(qst));
+    #endif
 }
 
 #define emit_native_mov_state_imm_via(emit, local_num, imm, reg_temp) \
@@ -301,6 +325,9 @@ STATIC void emit_native_start_pass(emit_t *emit, pass_kind_t pass, scope_t *scop
     emit->stack_size = 0;
     emit->const_table_cur_obj = 0;
     emit->const_table_cur_raw_code = 0;
+    #if MICROPY_PERSISTENT_CODE_SAVE
+    emit->qstr_link_cur = 0;
+    #endif
     emit->last_emit_was_return_value = false;
     emit->scope = scope;
 
@@ -598,6 +625,13 @@ STATIC void emit_native_end_pass(emit_t *emit) {
         emit->const_table = m_new(mp_uint_t, const_table_alloc);
         // Store mp_fun_table pointer just after qstrs
         emit->const_table[nqstr] = (mp_uint_t)(uintptr_t)mp_fun_table;
+
+        #if MICROPY_PERSISTENT_CODE_SAVE
+        size_t qstr_link_alloc = emit->qstr_link_cur;
+        if (qstr_link_alloc > 0) {
+            emit->qstr_link = m_new(mp_qstr_link_entry_t, qstr_link_alloc);
+        }
+        #endif
     }
 
     if (emit->pass == MP_PASS_EMIT) {
@@ -607,6 +641,11 @@ STATIC void emit_native_end_pass(emit_t *emit) {
         mp_emit_glue_assign_native(emit->scope->raw_code,
             emit->do_viper_types ? MP_CODE_NATIVE_VIPER : MP_CODE_NATIVE_PY,
             f, f_len, emit->const_table,
+            #if MICROPY_PERSISTENT_CODE_SAVE
+            emit->prelude_offset,
+            emit->const_table_cur_obj, emit->const_table_cur_raw_code,
+            emit->qstr_link_cur, emit->qstr_link,
+            #endif
             emit->scope->num_pos_args, emit->scope->scope_flags, 0);
     }
 }
@@ -1233,7 +1272,11 @@ STATIC void emit_native_import(emit_t *emit, qstr qst, int kind) {
 STATIC void emit_native_load_const_tok(emit_t *emit, mp_token_kind_t tok) {
     DEBUG_printf("load_const_tok(tok=%u)\n", tok);
     if (tok == MP_TOKEN_ELLIPSIS) {
+        #if MICROPY_PERSISTENT_CODE_SAVE
+        emit_native_load_const_obj(emit, MP_OBJ_FROM_PTR(&mp_const_ellipsis_obj));
+        #else
         emit_post_push_imm(emit, VTYPE_PYOBJ, (mp_uint_t)MP_OBJ_FROM_PTR(&mp_const_ellipsis_obj));
+        #endif
     } else {
         emit_native_pre(emit);
         if (tok == MP_TOKEN_KW_NONE) {