[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.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@224928 91177308-0d34-0410-b5e6-96231b3b80d8