Here's a very different patch. This gets rid of the __ptr_rel_ops and
just puts the special handling for pointers directly in the
std::less<_Tp*> etc. specializations. Then std::less<void> uses
std::less<P*> for some pointer type P*. I've also added a ton of ugly
metaprogramming to detect the "if the call operator calls a built-in
operator comparing pointers" condition. The idea is borrowed from
Casey Carter and Eric Niebler's Ranges work, but basically it checks
if operator<(T,U) or T.operator<(U) can be called, and if not the
comparison must be using a built-in operator. If both T and U are
convertible to pointers (specifically, to const volatile void* which
is the most accepting of all pointers) then we assume we're using a
built-in operator comparing pointers, and delegate to std::less<const
volatile void*>, which ensures a total order.

Tested powerpc64le-linux, committed to trunk. This fixes a regression,
but I'm not sure about backporting it yet, I haven't even tried
testing it on the branches.