[MIPS] Fix gn_unittests LoaderTest.Foo test

Test was failing with error message:
"Check failed: false. OS architecture not handled."

This change adds support for executing gn tool on mips platform.

BUG=130022
TEST=gn_unittests --gtest_filter=LoaderTest.Foo

Review URL: https://codereview.chromium.org/1890133002

Cr-Original-Commit-Position: refs/heads/master@{#387868}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 8ea3910c7dd3289ee7c49223c77032f0936d84bc
1 file changed
tree: 45574f1706459068490a3a8d0686c1a80cc0d69d
  1. tools/