libstdc++: Fix GDB Xmethod for std::shared_ptr::use_count() [PR109064]

libstdc++-v3/ChangeLog:

	PR libstdc++/109064
	* python/libstdcxx/v6/xmethods.py (SharedPtrUseCountWorker):
	Remove self-recursion in __init__. Add missing _supports.
	* testsuite/libstdc++-xmethods/shared_ptr.cc: Check use_count()
	and unique().