libstdc++: Re-enable some XPASS tests for AIX

libstdc++: Re-enable some XPASS tests for AIX

The deque shrink_to_fit.cc test always passes on AIX, I think it should
not have been disabled.

The 96088.cc tests pass for C++20 and later (I don't know why) so make
them require C++20, as they fail otherwise.

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/deque/capacity/shrink_to_fit.cc:
	Remove dg-xfail-run-if for AIX.
	* testsuite/23_containers/unordered_map/96088.cc: Replace
	dg-xfail-run-if with dg-require-effective-target c++20.
	* testsuite/23_containers/unordered_multimap/96088.cc: Likewise.
	* testsuite/23_containers/unordered_multiset/96088.cc: Likewise.
	* testsuite/23_containers/unordered_set/96088.cc: Likewise.