Enable snapshots on MIPS. This is based on

src/mips/builtins-mips.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1077 matching lines @@
   // Clear the context before we push it when entering the JS frame.
   __ mov(cp, zero_reg);
 
   // Enter an internal frame.
   {
     FrameScope scope(masm, StackFrame::INTERNAL);
 
     // Set up the context from the function argument.
     __ lw(cp, FieldMemOperand(a1, JSFunction::kContextOffset));
 
-    __ InitializeRootRegister();
-
     // Push the function and the receiver onto the stack.
     __ Push(a1, a2);
 
     // Copy arguments to the stack in a loop.
     // a3: argc
     // s0: argv, i.e. points to first arg
     Label loop, entry;
     __ sll(t0, a3, kPointerSizeLog2);
     __ addu(t2, s0, t0);
     __ b(&entry);
@@ skipping 686 matching lines @@
   __ bind(&dont_adapt_arguments);
   __ Jump(a3);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_MIPS