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.

This backport includes the fixes for the bootstrap error and the tests.

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>
(cherry picked from commit ac2fb60a67d6d1de6446c25c5623b8a1389f4770)