Linux apps are clang & GCC, macs use clang, windows is clang, gcc, and Microsoft's compilers. AFAIK, icc is pretty small in market share.
You think msvc, gcc and clang all have special optimizations for intel? Intel only controls their own compiler, icc.
GCC has `-march` and `-mtune` flags for arch-specific optimizations. Just because the compiler is agnostic doesn't mean it can't have, or welcome, arch-specific optimizations.