Reapply libcxxrt atomics cleanup commit, preparing for upstream fix
This reapplies upstream commit fd484be, as there is a follow-up fix for
the possible hangs in __cxa_guard_acquire() on i386:
Atomics cleanup (#11) We need to test exception specifiers but they're gone in C++17 so compile the tests with an older version of the standard. Rewrite the guard logic to be more idiomatic C++ and more comprehensible and make sure that atomics are used where necessary.
Obtained from: https://github.com/libcxxrt/libcxxrt/commit/fd484be
Fixes: 56aaed388b0a
MFC after: 2 weeks
(cherry picked from commit bfffb66e7f3cf9766e45a23080200c18535c2b85)