PATCH: Add bits/byteswap-16.h and use GCC builtin 32/64-bit bswap

[Roland McGrath]

You ignored my feedback about "16bit" in comments.

> +/* Macros to swap the order of bytes in 16bit integer values.
                                             ^
> +   Copyright (C) 2012  Free Software Foundation, Inc.
                        ^ excess space here

This is too much churn at once.  First do a change that just splits out
byteswap-16.h and nothing else.  Then we can look at the rest.


Thanks,
Roland