[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

tools/gn/args.cc

diff --git a/tools/gn/args.cc b/tools/gn/args.cc
index 60ee7b2..1034ea1 100644
--- a/tools/gn/args.cc
+++ b/tools/gn/args.cc
@@ -245,6 +245,7 @@
   static const char kX86[] = "x86";
   static const char kX64[] = "x64";
   static const char kArm[] = "arm";
+  static const char kMips[] = "mipsel";
   const char* arch = nullptr;
 
   // Set the host CPU architecture based on the underlying OS, not
@@ -256,6 +257,8 @@
     arch = kX64;
   else if (os_arch.substr(3) == "arm")
     arch = kArm;
+  else if (os_arch == "mips")
+    arch = kMips;
   else
     CHECK(false) << "OS architecture not handled.";