[lld] ELF: -r: Call assignAddresses only once (PR #152240)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> LGTM.
> 
> Could be worth an explanatory comment "Final addresses not available in relocatable links." but I'm OK without one.

Thanks! Switched to `while` and added a comment.

https://github.com/llvm/llvm-project/pull/152240