[PATCH] [libcxx] Add atomic_support.h header to src that handles needed atomic operations.

[Eric Fiselier]

In http://reviews.llvm.org/D10406#192273, @mclow.lists wrote:

> I really like that this stuff (support/atomic) is only used in the dylib.
>
> Does this patch fix: https://llvm.org/bugs/show_bug.cgi?id=22803  ?


Same patch. (It links here :P)

> And do we have an outstanding bug against `call_once` ?


No outstanding bug. The `call_once` changes are to fix/suppress a couple of TSAN diagnostics.  (See the TSAN buildbot)


http://reviews.llvm.org/D10406

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