Add support for the `avr-unknown-unknown` target to the compiler, which
enables the LLVM AVR target when available (should be compiled in by
default).

Implement the ABI, following the AVR GCC call convention, hence extern C
libraries must be compiled with AVR GCC (clang is buggy when passing of
returning some structs).

The ABI is experimental, and only basic calls have been verified against
the same calls made by AVR GCC so far. Both regular and "reduced tiny"
cores (with less registers) are supported.

This allows to know actual support for some features, for example which
pins are available.

Don't hardcode the alignment to 4 or 8 when the actual value depends on
the actual target (e.g. it's always 1 for AVR targets).

This reverts commit f1b5feb.

- Automatically link the executable program;
- Set file extension to ".elf" instead of none;
- Require CPU model (--mcpu) that impacts the ABI, codegen & linker

Co-authored-by: Sijawusz Pur Rahnama <sija@sija.pl>