Add support for xtensa and avr llvm embedded targets

[xfcasio]

Request to add xtensa (for esp boards) and avr (for arduino boards) llvm targets to c3c.

[lerno]

Xtensa is only available with LLVM 19-20. I've tried it and have LLVM crashes trying to run some simple code. I'll check in the result, but it's not usable yet.

AVR is a bit odd as it has multiple address spaces and 16 bit pointers. I'm not sure C3 can do that without some major additions (pointers with address spaces in particular)

[lerno]

Can we split these into three?

1. Xtensa
2. AVR
3. RiscV ESP boards

[xfcasio]

sounds like a good idea :+1:

[lerno]

Can you file them?

[lerno]

And then close this one.