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

src/mips64/assembler-mips64.h

Index: src/mips64/assembler-mips64.h
diff --git a/src/mips64/assembler-mips64.h b/src/mips64/assembler-mips64.h
index de09366b427865870ab2eed1860cc6a3f7f14501..6636d2b94a22e39c499fe1decd4cc50cbd523110 100644
--- a/src/mips64/assembler-mips64.h
+++ b/src/mips64/assembler-mips64.h
@@ -849,6 +849,9 @@ 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);