Implemented register to register instructions

src/build/arch/x64/ModRM.go

@@ -0,0 +1,9 @@
+package x64
+
+// ModRM is used to generate a ModRM suffix.
+// - mod: 2 bits
+// - reg: 3 bits
+// - rm:  3 bits
+func ModRM(mod byte, reg byte, rm byte) byte {
+	return (mod << 6) | (reg << 3) | rm
+}