This includes a structural change regarding atomic ops. Previously they
were enabled on all platforms unless we had knowledge that they did not
work. However both work performed by marius@ on sparc64 and the fact that
the 9.8.x branch is fussier in this area has demonstrated that this is
not a safe approach. So I've modified a patch provided by marius to
enable them for i386, amd64, and ia64 only.
