[cmake/multilib] Support multilib libdir suffixes by applying the

LLVM_LIBDIR_SUFFIX variable to one place in the cmake file.

This is all that I had to do to get everything from compiler-rt working
for me, but there may be more work required if folks are relying on more
parts of compiler-rt. Notably, I'm mostly using it for the sanitizers.

llvm-svn: 224928