Fix gm.py on Windows

Bug: v8:12131
Change-Id: Id3800e20b136c9fc16770e8a5d5c95e4674c0069
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3119380Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76587}

tools/dev/gm.py

@@ -28,6 +28,7 @@ not contain spaces.
 from __future__ import print_function
 import errno
 import os
+import platform
 import re
 import subprocess
 import sys
@@ -250,9 +251,7 @@ def _Notify(summary, body):
     print("{} - {}".format(summary, body))
 
 def _GetMachine():
-  # Once we migrate to Python3, this can use os.uname().machine.
-  # The index-based access is compatible with all Python versions.
-  return os.uname()[4]
+  return platform.machine()
 
 def GetPath(arch, mode):
   subdir = "%s.%s" % (arch, mode)