[libcxx] Fix compile for BUILD_EXTERNAL_THREAD_LIBRARY

Fix compilation with -DLIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY when using clang. Now linking target 'cxx_external_threads' with 'cxx-headers'. Fix mismatching visibility for libcpp_timed_backoff_policy function in file <__threading_support>.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D86598