If you look at the C language then there are two compilers: gcc and clang.
Clang uses LLVM as its back end, and is basically an Apple project. It is heavily optimized for arm64 (guess what: Apple uses arm64). LLVM is also the back end for Rust, Julia and many other languages. LuaJIT for example is pretty good on arm64 too.
Not true. Linaro is a thing, ARM also provides a first party compiler that is pretty good.
ARM has a team working on the arm64 GCC implementation so I expect it to be pretty well worked out like their own compiler.
When you look at hand optimised assembly in languages / library ARM is not even close to x64
and what exactly are you comparing to reach this conclusion: "and optimised assembly in languages / library ARM is not even close to x64"?