[llvm] [AVR] Force relocations for jumps and calls (PR #118015)

[Patryk Wychowaniec via llvm-commits]

Patryk27 wrote:

> Is there a strong reason for us to strictly follow avr-gcc?

No, I don't think so - I'm up for avoiding relocations as well 🙂 

> If you have no time for my suggested solution, I will try it later this month.

I can do that, no problem - I'm just reflecting on the approach:

- Option A: Create a new feature (like `RjmpWrap`) and enable it for the smaller (8 kB) microcontrollers,
- Option B: Teach LLVM flash sizes of _all_ of the AVRs and implement logic that automatically applies the memory wrapping trick.

Option A is simpler to implement, while option B (as described in the `Alternatives` approach at the top) is potentially more versatile (we'll be able to apply the wrapping trick even for larger microcontrollers, if we happen to generate a branch large enough). Note that for larger uCs this trick is just a program-size optimization, not a requirement.

I'm leaning towards option B, since it feels less hacky to me, but I don't have any strong preference here - care to choose? 

https://github.com/llvm/llvm-project/pull/118015