[lld] [LLD][ELF] add bp-* options in ELF (PR #120514)

[Fangrui Song via llvm-commits]

MaskRay wrote:

I refactored BPSectionOrderer with C++ CRTP #124482 . For Mach-O, rootSymbolToSectionIdxs is computed when iterating sections. 

For ELF, it would be more efficient to iterate over symbols and populate rootSymbolToSectionIdxs. symbolToSectionIdxs can be removed.

https://github.com/llvm/llvm-project/pull/120514