[libcxx] Do not declare the thread api when __external_threading is present

This fixes a small omission where even when __external_threading is provided,
we attempt to declare a pthread based threading API. Instead, we should leave
out everything for the __external_threading header to take care of.

The __threading_support header provides a proof-of-concept externally threaded
libc++ variant when _LIBCPP_HAS_THREAD_API_EXTERNAL is defined. But if the
__external_threading header is present, we should exclude all of that POC stuff.

Reviewers: EricWF

Differential revision: https://reviews.llvm.org/D25468

llvm-svn: 284232