X86: Implement the stul microop.

This microop does a store and unlocks the requested address. The RISC86
microop ISA doesn't seem to have an equivalent to this, so I'm guessing that
the store following an ldstl is automatically unlocking. We don't do it this
way for performance reasons since the behavior is the same.

src/arch/x86/isa/microops/ldstop.isa

@@ -478,6 +478,8 @@ let {{
         microopClasses[name] = StoreOp
 
     defineMicroStoreOp('St', 'Mem = pick(Data, 2, dataSize);')
+    defineMicroStoreOp('Stul', 'Mem = pick(Data, 2, dataSize);',
+            mem_flags="Request::LOCKED")
     defineMicroStoreOp('Stfp', 'Mem = FpData.uqw;')
     defineMicroStoreOp('Stupd', 'Mem = pick(Data, 2, dataSize);',
             'Base = merge(Base, EA - SegBase, addressSize);',