libstdc++: Move std::monostate to <utility> for C++26 (P0472R2)

libstdc++: Move std::monostate to <utility> for C++26 (P0472R2)

Another C++26 paper just approved in Wrocław. The std::monostate class
is defined in <variant> since C++17, but for C++26 it should also be
available in <utility>.

libstdc++-v3/ChangeLog:

	* include/Makefile.am: Add bits/monostate.h.
	* include/Makefile.in: Regenerate.
	* include/std/utility: Include <bits/monostate.h>.
	* include/std/variant (monostate, hash<monostate>): Move
	definitions to ...
	* include/bits/monostate.h: New file.
	* testsuite/20_util/headers/utility/synopsis.cc: Add monostate
	and hash<monostate> declarations.
	* testsuite/20_util/monostate/requirements.cc: New test.