[libcxx] Fix _LIBCPP_HAS_THREAD_API_EXTERNAL build

Summary:
The definition of `__libcpp_timed_backoff_policy` and the declaration of
`__libcpp_thread_poll_with_backoff` must not be guarded by
  #if !defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
because the definitions of `__libcpp_timed_backoff_policy::operator()`
and `__libcpp_thread_poll_with_backoff` aren't guarded by this macro
(and this is correct because these two functions are implemented in
terms of other libc++ functions and don't interact with the host
threading library).

Reviewers: ldionne, __simt__, EricWF, mclow.lists

Reviewed By: ldionne

Subscribers: dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D75191