AVR: Support C backend

[ikskuh]

Right now, the AVR support is degraded. With using avr-gcc, we can make it work quite well though.

This requires updating the AVR code to use a different logic in the start code, and just implement a extern fn main() c_int, that jumps into the microzig code and is happy about .bss, .data being already set up.

As-is, we cannot implement the whole startup logic ourselves via C backend, as some features are still missing.