libstdc++: Ensure pool resources meet alignment requirements [PR118681]

libstdc++: Ensure pool resources meet alignment requirements [PR118681]

For allocations with size > alignment and size % alignment != 0 we were
sometimes returning pointers that did not meet the requested aligment.
For example, allocate(24, 16) would select the pool for 24-byte objects
and the second allocation from that pool (at offset 24 bytes into the
pool) is only 8-byte aligned not 16-byte aligned.

The pool resources need to round up the requested allocation size to a
multiple of the alignment, so that the selected pool will always return
allocations that meet the alignment requirement.

libstdc++-v3/ChangeLog:

	PR libstdc++/118681
	* src/c++17/memory_resource.cc (choose_block_size): New
	function.
	(synchronized_pool_resource::do_allocate): Use choose_block_size
	to determine appropriate block size.
	(synchronized_pool_resource::do_deallocate): Likewise
	(unsynchronized_pool_resource::do_allocate): Likewise.
	(unsynchronized_pool_resource::do_deallocate): Likewise
	* testsuite/20_util/synchronized_pool_resource/118681.cc: New
	test.
	* testsuite/20_util/unsynchronized_pool_resource/118681.cc: New
	test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>