The std::cbegin function in <bits/range_access.h> can't call the
std::begin function in <valarray> unless it knows about it, so we need
to declare the valarray overloads in <bits/range_access.h>.

I considered adding a <bits/valarray_fwd.h> header with the
declarations, but I don't think that's worth it, so I'm just
declaring them directly in range_access.h

Tested powerpc64le-linux, committed to trunk. I'll probably commit to
the gcc-5 branch soon too.