libstdc++: Implement C++23 std::bind_back from P2387R3 [PR108827]

The implementation is based off of std::bind_front.  Since this is a
C++23 feature we use deducing this unconditionally.

	PR libstdc++/108827
	PR libstdc++/111327

libstdc++-v3/ChangeLog:

	* include/bits/version.def (bind_back): Define.
	* include/bits/version.h: Regenerate.
	* include/std/functional (_Bind_back): Define for C++23.
	(bind_back): Likewise.
	* testsuite/20_util/function_objects/bind_back/1.cc: New test
	(adapted from corresponding bind_front test).
	* testsuite/20_util/function_objects/bind_back/111327.cc: Likewise.