[postmortem] Load files using utf-8 to support Python 3

Change-Id: I174d38cc33210c07d1a7596627e1b2d21bb06313
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1895560Reviewed-by: Mathias Bynens <mathias@chromium.org>
Commit-Queue: Michaël Zasso <mic.besace@gmail.com>
Cr-Commit-Position: refs/heads/master@{#64717}

tools/gen-postmortem-metadata.py

@@ -49,6 +49,7 @@
 # for py2/py3 compatibility
 from __future__ import print_function
 
+import io
 import re
 import sys
 
@@ -376,7 +377,7 @@ def load_objects():
 
 
 def load_objects_from_file(objfilename, checktypes):
-        objfile = open(objfilename, 'r');
+        objfile = io.open(objfilename, 'r', encoding='utf-8');
         in_insttype = False;
         in_torque_insttype = False
 
@@ -585,7 +586,7 @@ def load_fields():
 
 
 def load_fields_from_file(filename):
-        inlfile = open(filename, 'r');
+        inlfile = io.open(filename, 'r', encoding='utf-8');
 
         #
         # Each class's fields and the corresponding offsets are described in the