[v3] libstdc++/28671

[Paolo Carlini]

Benjamin Kosnik wrote:

>This removes inlined helper function definitions from atomicity.h,
>restoring the behavior present in 3.2-4.1 libstdc++.
>  
>
Why this heavy hand? Only because of the x86 quirks we are penalizing 
everything else and now we have to reopen libstdc++/24692.

If we change the test for _GLIBCXX_ATOMIC_BUILTINS to be smarter (in 
particular, it can simply be skipped whenever that target provides an 
implementation of the atomic functions) then we can fix the issue in a 
better way.

Paolo.