Properly set EMAR for Interprocedural Optimization

[Skylion007]

This properly sets the EMAR for InterproceduralOptimization as described here: https://github.com/emscripten-core/emscripten/issues/11143 and here https://github.com/mosra/magnum/issues/490

[Squareys]

Looks great, thanks @Skylion007 !

@mosra FYI, I do the -flto for Wonderland Engine, too (though, I set those at project level, not for compiling magnum). And faintly remember setting em-ar somewhere also.

[Skylion007]

Ping @mosra

[mosra]

This was finally merged in 98bdb68cb17d9c70d4ba179509075ba479764a5b and b25ca8905d48a5d380eda95dd1a299216a3cad78. The extra delay allowed me to not have to care about fastcomp compatibility anymore.