This allows CGo code to call some libc functions. Additionally, by
putting memset/memmove/memcpy in an archive they're not included anymore
when not necessary, reducing code size for small programs.
We don't need the separate submodule: compiler-rt is already included in
the llvm-project repository.
This should hopefully make CI slightly faster too.
These files don't really belong in this repository. It's better to
generate them automatically from a source, like the one provided by the
avr-rust project. So a new command `make gen-device-avr` has been
provided for this purpose.