pmr::resource_adaptor can avoid allocating an oversized buffer and doing
manual alignment within that buffer when the wrapped allocator is known
to always meet the requested alignment. Specifically, if the allocator
is known to use malloc or new directly, then we can call the allocator
directly for any fundamental alignment.

	PR libstdc++/70940
	* include/experimental/memory_resource
	(__resource_adaptor_common::_AlignMgr::_M_unadjust): Add assertion.
	(__resource_adaptor_common::__guaranteed_alignment): New helper to
	give maximum alignment an allocator guarantees. Specialize for known
	allocators using new and malloc.
	(__resource_adaptor_imp::do_allocate): Use __guaranteed_alignment.
	(__resource_adaptor_imp::do_deallocate): Likewise.
	* testsuite/experimental/memory_resource/new_delete_resource.cc:
	Check that new and delete are called with expected sizes.

This wouldn't be a very important optimization, because wrapping
std::allocator in a pmr::resource_adaptor is probably uncommon, but
pmr::resource_adaptor<new_allocator> is used to implement
pmr::new_delete_resource() in <experimental/memory_resource>. (The
C++17 version of pmr::new_delete_resource() just uses aligned new, but
that isn't available in C++14).

Tested powerpc64le-linux, committed to trunk.

Rainer, this is another place where alignof(max_align_t) gets encoded
into the ABI, so is affected by PR 77691 as well.