Add vendor macro to "lld"

Summary:
Add the vendor macro to "lld" for extended version output support,
such that it's able to print additional version info. This is
consistent with the Clang and LLVM version printer, and the
additional version message can be provided via PACKAGE_VENDOR.

Reviewers: hubert.reinterpretcast, kbarton, cebowleratibm, rzurob, ruiu

Reviewed By: hubert.reinterpretcast

Subscribers: emaste, mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79026