[MIPS] Fix gn_unittests LoaderTest.Foo test

tools/gn/args.cc

Index: tools/gn/args.cc
diff --git a/tools/gn/args.cc b/tools/gn/args.cc
index 60ee7b2ce33d385f58957c9f01858536a3c4d147..1034ea1f31f92ba7015dcfacf68dd767d461519d 100644
--- a/tools/gn/args.cc
+++ b/tools/gn/args.cc
@@ -245,6 +245,7 @@ void Args::SetSystemVarsLocked(Scope* dest) const {
   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 @@ void Args::SetSystemVarsLocked(Scope* dest) const {
     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.";