[toolchain] Enable LLD, LTO and minsize by default

[pamaury]

Now that many targets depend on the DT, LTO has become a lot more important. Since we already compile critical piece of software with it (test_rom, mask_rom), it makes sense to enable it for everything.

NOTE: I don't know if this going to break stuff, our guardian CI will surely let us know.

[pamaury]

Fixes #26352

[jwnrt]

Thanks for figuring this out, it will be very helpful.

For posterity, those shift functions were removed because of concerns about the type punning in the union. We know it's "correct" because we know the representation on Ibex, but we weren't sure if it's UB and could allow that compiler to miscompile. Is that a correct summary @pamaury?

[pamaury]

Thanks for figuring this out, it will be very helpful.

For posterity, those shift functions were removed because of concerns about the type punning in the union. We know it's "correct" because we know the representation on Ibex, but we weren't sure if it's UB and could allow that compiler to miscompile. Is that a correct summary @pamaury?

I am not an expert on the C specification but my understanding is that type punning in C99 is a priori not undefined behaviour, although the standard is quite weak on that point. However, unless one knows what the actual representation of integers are, this is still UB. One could say that this is fixed by C23 that set the representation to two's complement but we are not compiling for C23. That being said, as you noted, we know that we are compiling for the Ibex so there is no ambiguity here about the representation.