Build esmdk's compiler binaries with PGO

Hi!

Clang [supports](https://llvm.org/docs/HowToBuildWithPGO.html) building with Profile-Guided Optimization (PGO), and it helps with achieving +20% compilation speed. I think would be a good idea to try to build Emscripten itself with PGO. I've performed searching across the repository about it but didn't find the results. If you already have any - please share them with me.

Thanks in advance!