FatELF: universal binaries for Linux

"Relatedly, if compilers and linkers are FatELF-aware, they can build fatELFs a bit faster because they only have to do the parsing once, and then object code for all the platforms from the same resultant parse tree."

Unfortunately, that's not true in any language where the use of a preprocessor is standard and commonplace, like C or C++. The preprocessor is a step run before parsing, and the work the preprocessor does may change what the result of parsing is by adding/removing/changing parts of the program text. It's very common in C/C++ to use the preprocessor to handle things like type selection, type size and byte order issues - exactly the sorts of things that'll change depending on target arch.