libstdc++: handle small max_blocks_per_chunk in pool resources [PR 94160]

libstdc++: handle small max_blocks_per_chunk in pool resources [PR 94160]

When a pool resource is constructed with max_blocks_per_chunk=1 it ends
up creating a pool with blocks_per_chunk=0 which means it never
allocates anything. Instead it returns null pointers, which should be
impossible.

To avoid this problem, round the max_blocks_per_chunk value to a
multiple of four, so it's never smaller than four.

libstdc++-v3/ChangeLog:

	PR libstdc++/94160
	* src/c++17/memory_resource.cc (munge_options): Round
	max_blocks_per_chunk to a multiple of four.
	(__pool_resource::_M_alloc_pools()): Simplify slightly.
	* testsuite/20_util/unsynchronized_pool_resource/allocate.cc:
	Check that valid pointers are returned when small values are
	used for max_blocks_per_chunk.