[PATCH] support: user more portable atomic wrappers

[Florian Weimer]

* Vineet Gupta:

> I agree that gcc atomics should be baseline. I would still propose to
> carry this patch as it makes code less verbose if nothing else and the
> wrappers are part of glibc already.

The advantage of the GCC built-ins is that they have easy-to-find
documentation.  Our wrapper macros are in the same file as the legacy
atomics that no longer should be used.  They are also not easy to use
correctly (e.g., there is no support for access to single bytes, but the
code will still compile on some architectures).

The GCC built-ins have other problems, of course.  For example, one
might accidentally introduce a dependency on libatomic (leading to a
link failure later).  But I think these issues are less severe.

Thanks,
Florian
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