Add symlink to LLVM

[SergioGasquez]

In Unix systems, it creates a symlink from $HOME/.espup/esp-clang/ to the directory containing Xtensa clang. This will help std projects, as embuild, could check if LIBCLANG_PATH is set by the user and if its not, it will be defaulted to $HOME/.espup/esp-clang, with this we can avoid the usual error of the user forgetting to source the export script.

This is only helpful in std approach, for no_std it does nothing