Eo: Implement the fallback eo_add implementation.

Eo: Implement the fallback eo_add implementation.

The current eo_add uses a (very useful) gcc extension that is only
available in gcc compatible compilers (e.g clang). Until this commit we
just temporarily ignored this fact. This adds a fallback implementation that
can be used interchangeably with the non portable one. This means that the
same binary can call either at any point in time and the code will work.

Breaks ABI.