[CMake] Fix linker detection in AddLLVM.cmake

Fix linker not being correctly detected when a custom one is specified
through LLVM_USE_LINKER CMake variable.

In particular,

  cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_USE_LINKER=gold ../llvm

resulted into

  Linker detection: GNU ld

instead of

  Linker detection: GNU Gold

due to the construction not accounting for such variable. It led to the general
confusion and prevented setting linker-specific flags inside functions defined
in AddLLVM.cmake.

Thanks Oleksii Vilchanskyi for the patch!

llvm-svn: 316956