PR libstdc++/88111 Make maximum block size depend on size_t width

PR libstdc++/88111 Make maximum block size depend on size_t width

	PR libstdc++/88111
	* include/std/memory_resource (pool_options): Add Doxygen comments.
	* src/c++17/memory_resource.cc (pool_sizes): Only use suitable values
	on targets with 16-bit or 20-bit size_t type.
	(munge_options): Make default values depend on width of size_t type.

From-SVN: r266353