MIPS: [Atomics] Remove Atomics code stubs; use TF ops.

src/mips/assembler-mips.h

Index: src/mips/assembler-mips.h
diff --git a/src/mips/assembler-mips.h b/src/mips/assembler-mips.h
index 886ac6c0525ef1ad170f269c0a9218cc58b16f36..ed5cbf8bf2ee2fd6844572d1b4097f8f41e72718 100644
--- a/src/mips/assembler-mips.h
+++ b/src/mips/assembler-mips.h
@@ -799,6 +799,9 @@ class Assembler : public AssemblerBase {
   void teq(Register rs, Register rt, uint16_t code);
   void tne(Register rs, Register rt, uint16_t code);
 
+  // Memory barrier instruction.
+  void sync();
+
   // Move from HI/LO register.
   void mfhi(Register rd);
   void mflo(Register rd);