libsupc++: Change _Unordered comparison value to minimum value of signed char.

libsupc++: Change _Unordered comparison value to minimum value of signed char.

For any minimum value of a signed type, its negation (with wraparound) results
in the same value, behaving like zero. Representing the unordered result with
this minimum value, along with 0 for equal, 1 for greater, and -1 for less
in partial_ordering, allows its value to be reversed using unary negation.

The operator<=(partial_order, 0) now checks if the reversed value is positive.
This works correctly because the unordered value remains unchanged and thus
negative.

libstdc++-v3/ChangeLog:

	* libsupc++/compare (_Ncmp::_Unordered): Rename and change the value
	to minimum value of signed char.
	(_Ncomp::unordered): Renamed from _Unordered, the name is reserved
	by partial_ordered::unordered.
	(partial_ordering::_M_reverse()): Define.
	(operator<=(partial_ordering, __cmp_cat::__unspec))
	(operator>=(__cmp_cat::__unspec, partial_ordering)): Implemented
	in terms of negated _M_value.
	(operator>=(partial_ordering, __cmp_cat::__unspec))
	(operator<=(__cmp_cat::__unspec, partial_ordering)): Directly
	compare _M_value, as unordered value is negative.
	(partial_ordering::unordered): Handle _Ncmp::unoredred rename.
	* python/libstdcxx/v6/printers.py: Add -128 as integer value
	for unordered, keeping 2 to preserve backward compatibility.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>