AIX atomicity.h support

[David Edelsohn]

Ben,

	As soon you you decide how libstdc++-v3 is going to provide the
structure to allow the problems encountered by AIX and other non-GNU/Linux
systems to be handled, I can work on the patches.  I have been trying to
open a dialogue on the facilities that are needed for portability.  I do
not have time to work on AIX patches or libstdc++-v3 restructuring without
some dialogue and guidance.

David