[lld] [lld][elf] add safe-thunks in ELF (PR #126695)

[Fangrui Song via llvm-commits]

MaskRay wrote:

The safe_thunks mode (a more canonical spelling should be `--icf=safe-thunks`) introduces a small size benefit, but adds complexity that may outweigh the gains.
Honestly I am not certain we should add this.

Regarding the range extension thunk framework: The current implementation, using an R_AARCH64_JUMP26 relocation within `InputSection::relocations`, actually works transparently.
However, testing this requires a code section of at least 128MiB, which is not good (
ICF eligible sections must be in the same output section (https://reviews.llvm.org/D66717).
To test --icf=safe_thunks triggering thunks, we need two input sections, placed in one output section that is at least 128MiB large.)

With address information, we could replace `folded: b foo_thunk; ... foo_thunk: adrp; add; br x16` with `folded: adrp; add; br x16`, but this would probably 
significantly increase the complexity of that already intricate code, impacting long-term maintainability for what is a relatively minor feature.
We could also place identical functions together in runBalancedPartitioning, with some code complexity.

Frankly, I am not convinced that adding this safe_thunks feature is the right move.

(Pasted to https://discourse.llvm.org/t/rfc-add-icf-safe-thunks-in-elf-like-macho/84584)

https://github.com/llvm/llvm-project/pull/126695