libstdc++: add constexpr stable_partition

libstdc++: add constexpr stable_partition

This completes the implementation of P2562R1 for C++26.

Unlike the other constexpr algorithms of the same family,
stable_partition does not have a constexpr-friendly version "ready to
use" during constant evaluation. In fact, it is not even available on
freestanding, because it always allocates a temporary memory buffer.

This commit implements the simplest possible strategy: during constant
evaluation allocate a buffer of length 1 on the stack, and use that as
a working area.

libstdc++-v3/ChangeLog:

	* include/bits/algorithmfwd.h (stable_partition): Mark it
	as constexpr for C++26.
	* include/bits/ranges_algo.h (__stable_partition_fn): Likewise.
	* include/bits/stl_algo.h (stable_partition): Mark it as
	constexpr for C++26; during constant evaluation use a new
	codepath where a temporary buffer of 1 element is used.
	* testsuite/25_algorithms/headers/algorithm/synopsis.cc
	(stable_partition): Add constexpr.
	* testsuite/25_algorithms/stable_partition/constexpr.cc: New test.