libstdc++: Fix std::shared_ptr pretty printer for GDB 11

libstdc++: Fix std::shared_ptr pretty printer for GDB 11

This pretty printer was updated for GCC 16 to match a change to
std::atomic<shared_ptr<T>>.  But the gdb.Type.is_scalar property was
added in GDB 12.1, so we get an error for older GDB versions.

This adds a workaround for older GDB versions. The gdb.Type.tag property
is None for scalar types, and should always be defined for the
std::atomic class template. Another option would be to use the
is_specialization_of function defined in printers.py, but just checking
for the tag is simpler.

libstdc++-v3/ChangeLog:

	* python/libstdcxx/v6/printers.py (SharedPointerPrinter): Only
	use gdb.Type.is_scalar if supported.
	* testsuite/libstdc++-prettyprinters/compat.cc: Test printer for
	old implementation of std::atomic<std::shared_ptr<T>>.

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