[lld] [lld-macho,BalancedPartition] Simplify relocation hash and avoid xxHash (PR #121729)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> > > > @ellishg Does the current PR look good to you?
> > > 
> > > 
> > > Can we use only `xxh3_64bits()` in this PR for simplicity and flexibility?
> > 
> > 
> > Do you want two PRs, one changing xxHash64 to `xxh3_64bits`, and the other simplifying the relocation hash?
> > If this is your intention, I can pre-land one commit to make just the xxHash64 change, and rebase this PR.
> 
> One PR should be ok. I was just saying we should use `xxh3_64bits()` instead of `read32le()` on line 100.

Using a hash function like `xxh3_64bits` for the k-mers introduces unnecessary performance overhead...
It's ok to have two sets of values into `hashes`: (a) instructions/data (b) `xxh3_64bits` derived values (relocations).

Well, for this PR to proceed I could give up but I would not let the ELF port take this slight performance overhead here...

https://github.com/llvm/llvm-project/pull/121729