Merge libcxxrt c812a07cd2f95c1403baf0bbe0366e7618d1d6d3:

* Don't call the _fast version of the TLS accessor in terminate() or
  unexpected().
  1) TLS may not have been set up yet.
  2) When we're in one of these functions, Really Bad Stuff has
     happened and potentially saving a few cycles really isn't
     important.
* Merge in fixes from FreeBSD trunk to make atomics work with recent
  clang.

MFC after:	1 week