[MLIR][GPU][lld] Use LLD bundled in ROCm, removing workaround

Having clarified that executing the SerializeToHsaco pass can
depend on a ROCm installation, switch from calling lld as a library to
using the copy of lld guaranteed to be included in a ROCm install.

This removes the workaround introduced in D119277

Reviewed By: whchung

Differential Revision: https://reviews.llvm.org/D119463