[lld] [lld][ELF] Add --why-live flag (inspired by Mach-O) (PR #127112)

[Peter Smith via llvm-commits]

smithp35 wrote:

> > Finally, one that you may want to exclude, is any Arm/AArch64 mapping symbol from being included in the why-live output. These often are defined at the same address as function and data symbols, but you'll almost certainly want to prefer the function and data symbol.
> 
> Only sized or directly referenced symbols can be reported as liveness reasons. Given that the mapping symbols mark positions, I'd assume they wouldn't have either of those characteristics. Is that correct?

Yes that is correct. They aren't directly called and they have no size.

https://github.com/llvm/llvm-project/pull/127112