[multilib] Teach LLD's CMake build to use LLVM_LIBDIR_SUFFIX which
allows it to support multilib suffixed hosts using lib64, etc. This
variable is now available both in the direct LLVM build and from the
LLVMConfig.cmake file used by standalone builds.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@224925 91177308-0d34-0410-b5e6-96231b3b80d8