tools/mpy-tool.py: Fix linking of qstr objects in native ARM Thumb code.

Previously, when linking qstr objects in native code for ARM Thumb, the
index into the machine code was being incremented by 4, not 8.  It should
be 8 to account for the size of the two machine instructions movw and movt.
This patch makes sure the index into the machine code is incremented by the
correct amount for all variations of qstr linking.

See issue #4829.

tools/mpy-tool.py

@@ -481,6 +481,7 @@ class RawCodeNative(RawCode):
         if kind == 0:
             # Generic 16-bit link
             print('    %s & 0xff, %s >> 8,' % (qst, qst))
+            return 2
         else:
             # Architecture-specific link
             is_obj = kind == 2
@@ -488,14 +489,17 @@ class RawCodeNative(RawCode):
                 qst = '((uintptr_t)MP_OBJ_NEW_QSTR(%s))' % qst
             if config.native_arch in (MP_NATIVE_ARCH_X86, MP_NATIVE_ARCH_X64):
                 print('    %s & 0xff, %s >> 8, 0, 0,' % (qst, qst))
+                return 4
             elif MP_NATIVE_ARCH_ARMV6M <= config.native_arch <= MP_NATIVE_ARCH_ARMV7EMDP:
                 if is_obj:
                     # qstr object, movw and movt
                     self._asm_thumb_rewrite_mov(pc, qst)
                     self._asm_thumb_rewrite_mov(pc + 4, '(%s >> 16)' % qst)
+                    return 8
                 else:
                     # qstr number, movw instruction
                     self._asm_thumb_rewrite_mov(pc, qst)
+                    return 4
             else:
                 assert 0
 
@@ -523,8 +527,7 @@ class RawCodeNative(RawCode):
                 # link qstr
                 qi_off, qi_kind, qi_val = self.qstr_links[qi]
                 qst = global_qstrs[qi_val].qstr_id
-                self._link_qstr(i, qi_kind, qst)
-                i += 4
+                i += self._link_qstr(i, qi_kind, qst)
                 qi += 1
             else:
                 # copy machine code (max 16 bytes)