Aarch64: Use stp to initialize object on C1

It is found that in C1_MacroAssembler::initialize_object which we can use stp to replace str.
@@ -250,13 +249,16 @@ void C1_MacroAssembler::initialize_object(Register obj, Register klass, Register
        for (int i = -unroll; i < 0; i++) {
          if (-i == remainder)
            bind(entry_point);
- str(zr, Address(rscratch1, i * wordSize));
+ stp(zr, zr, Address(rscratch1, i * 2 * BytesPerWord));