libstdc++: Implement std::strong_order for floating-point types [PR96526]

libstdc++: Implement std::strong_order for floating-point types [PR96526]

This removes a FIXME in <compare>, defining the total order for
floating-point types. I originally opened PR96526 to request a new
compiler built-in to implement this, but now that we have std::bit_cast
it can be done entirely in the library.

The implementation is based on the glibc definitions of totalorder,
totalorderf, totalorderl etc.

I think this works for all the types that satisfy std::floating_point
today, and should also work for the types expected to be added by P1467
except for std::bfloat16_t. It also supports some additional types that
don't currently satisfy std::floating_point, such as __float80, but we
probably do want that to satisfy the concept for non-strict modes.

libstdc++-v3/ChangeLog:

	PR libstdc++/96526
	* libsupc++/compare (strong_order): Add missing support for
	floating-point types.
	* testsuite/18_support/comparisons/algorithms/strong_order_floats.cc:
	New test.