[NFC][ELF] Don't duplicate DynamicReloc constructor
This second constructor is just a shorthand for an AddendOnly relocation
against dummySym with R_ADDEND, so write it as such.
Reviewers: arichardson, MaskRay
Reviewed By: MaskRay, arichardson
Pull Request: https://github.com/llvm/llvm-project/pull/150811
