Configure: mixed-endian double-doubles

Configure: mixed-endian double-doubles

The ppc64el is the first seen little-endian double-double (and also
the first little-endian ppc), but it turns out its little-endianness
is mixed: the doubles are still in big-endian order.  Configure was
expecting wrongly a fully byte-reversed double-double.

Therefore extend the long double format detection to cover all the
(double-double) permutations, though the formats of five and eight
are rather unlikely (based on current platforms using double-double).