[compiler-rt] Don't use libcxx if it's not enabled by LLVM_ENABLE_PROJECTS

This fixes sanitizer-x86_64-linux-fuzzer which switched to new layout and not it
can't link as libcxx is always there.
We should support and tests libcxx but still it was unexpected that libfuzzer
ignored LLVM_ENABLE_PROJECTS_USED.

llvm-svn: 370153