Fix JS_ReadString for wide strings on big endian targets

[chqrlie]

wide characters are stored in little endian order, so they must be swapped when read in a big endian target

[saghul]

Any chance of a test?

[chqrlie]

Any chance of a test?

To test the serialization of strings:

• use a wide character in example/hello.js and check for its presence at runtime when running the executable

More generally:

• we could test the serialization on both little-endian and big-endian systems passing data to workers.

[chqrlie]

I am going to merge this patch. Adding a test requires a better understanding of the CI tests and the cmake stuff, so I am not the best candidate for this.