libstdc++: Fix __normal_iterator comparisons for C++20

libstdc++: Fix __normal_iterator comparisons for C++20

This fixes a regression introduced when I replaced __normal_iterator's
relational operators with operator<=>. If the wrapped iterator type
doesn't define operator<=> then __normal_iterator doesdn't either, which
breaks any use of fancy pointers that don't define <=>. The regression
was found when trying to build cmcstl2.

The solution is to use synth-three-way to define __normal_iterator's
spaceship operator, so that it is still defined even if the wrapped type
only supports operator<.

	* include/bits/stl_iterator.h (__normal_iterator): Use synth-three-way
	to define operator<=>.
	* testsuite/24_iterators/normal_iterator/cmp_c++20.cc: New test.