[lld] Make hashtable entry size configurable (PR #113431)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Thanks for the patch, but is this needed? `.hash` is quite obsoleted (https://maskray.me/blog/2022-08-21-glibc-and-dt-gnu-hash)

We could make a special case to make `--hash-style=both` mean just `gnu` for SystemZ. `sysv` could be made unsupported.

https://github.com/llvm/llvm-project/pull/113431