C++ target doesn't support strings with surrogate pairs

https://github.com/jtransc/jtransc/blob/7dcdac0a6dfb7e23f0f005d07af4fe47ac4b214c/jtransc-main/test/javatest/nio/ModifiedUtf8Test.java#L14

//expect(new byte[]{0, 15, 104, 101, 108, 108, 111, -64, -128, -62, -128, -19, -81, -65, -19, -65, -65}, "hello\0\u0080\uDBFF\uDFFF"); // Surrogate pairs