[CMake] Only detect the linker once in AddLLVM.cmake

Summary:
Otherwise, the build output contains a bunch of "Linker detection: <xxx>"
lines that are really redundant. We also make redundant calls to the
linker, although that is a smaller concern.

Reviewers: smeenai

Subscribers: mgorny, fedor.sergeev, jkorous, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68648