[lld] Remove special cases from default ld driver mode.

Summary:
Use the Gnu driver mode by default for all platforms when ld is
invoked. Other names for the program (such as link or ld64) continue
working as before.

Reviewers: MaskRay, int3, srhines, smeenai, ruiu

Reviewed By: MaskRay, srhines, smeenai, ruiu

Subscribers: smeenai, srhines, nickdesaulniers, llvm-commits

Tags: #lld, #llvm

Differential Revision: https://reviews.llvm.org/D78837