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

[Marshall Clow]

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  ?

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


http://reviews.llvm.org/D10406

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