[PATCH] Add replacement endian.h and byteswap.h to libgnu

[Florian Weimer]

* Ulf Hermann:

> Some systems don't provide endian.h and byteswap.h. The required
> functions are trivial to define using sys/param.h and gcc builtins,
> though.
>
> Also, include endian.h in dwelf_scn_gnu_compressed_size.c as that uses
> be64toh().

This is still an issue with non-glibc, non-BSD compilation.  The patch
is not ideal, as it depends on a GCC extension, but it's an
improvement for those who use GCC on a platform which does not provide
these functions as part of the C library.