LLVM Version Harmonization

[Victor-Jung]

Currently, we use LLVM 12 to compile applications for the Snitch platform, while we use LLVM 15 for the rest. We should investigate if there is a strong reason not to use LLVM 15 everywhere.

[FrancescoConti]

Does it make sense to do this and then switch afterwards to LLVM 18 when it's ready? Or better to do everything in one step?

[Victor-Jung]

Good point; I think it's best to try harmonizing the version when we move to 18.

[Victor-Jung]

Implemented in #60