tree 91db782cf133d6b4dc63576a75b9e16bf97a3a18
parent f4338f92d1eb7ed8f8abdae1b1ea908f64c7be64
author Louis Dionne <ldionne.2@gmail.com> 1637164771 -0500
committer Copybara-Service <copybara-worker@google.com> 1637208403 -0800

[libc++] Enable <atomic> when threads are disabled

std::atomic is, for the most part, just a thin veneer on top of compiler
builtins. Hence, it should be available even when threads are not available
on the system, and in fact there has been requests for such support.

This patch:
- Moves __libcpp_thread_poll_with_backoff to its own header so it can
  be used in <atomic> when threads are disabled.
- Adds a dummy backoff policy for atomic polling that doesn't know about
  threads.
- Adjusts the <atomic> feature-test macros so they are provided even when
  threads are disabled.
- Runs the <atomic> tests when threads are disabled.

rdar://77873569

Differential Revision: https://reviews.llvm.org/D114109

GitOrigin-RevId: 92832e4889ae6038cc8b3e6e449af2a9b9374ab4