Reland D78837 [lld] Remove special cases from default ld driver mode.

Drops the behavior from rL217112.

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.

Reviewed By: MaskRay, srhines, smeenai, ruiu

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