libstdc++: Only use std::atomic<tzdb_list::_Node*> if lock free [PR108228]

libstdc++: Only use std::atomic<tzdb_list::_Node*> if lock free [PR108228]

This fixes linker errors for hppa-hp-hpux11.11 due to an undefined weak
symbol and the use of atomic operations that require libatomic.

The weak symbol can simply be defined, which we already do for darwin.

The std::atomic<_Node*> is only an optimization, so can be avoided for
targets where the underlying atomic ops aren't available without help
from libatomic. The accesses to the std::atomic<_Node*> can be
abstracted behind a new API for getting and setting the cached value,
and then the atomics can be used conditionally.

libstdc++-v3/ChangeLog:

	PR libstdc++/108228
	PR libstdc++/108235
	* config/abi/pre/gnu.ver: Move zoneinfo_dir_override export to
	the latest symbol version.
	* src/c++20/tzdb.cc (USE_ATOMIC_SHARED_PTR): Define to 0 if
	atomic<_Node*> is not always lock free.
	(USE_ATOMIC_LIST_HEAD): New macro.
	[__hpux__] (__gnu_cxx::zoneinfo_dir_override()): Provide
	definition of weak symbol.
	(tzdb_list::_Node::_S_head): Rename to _S_head_cache.
	(tzdb_list::_Node::_S_list_head): New function for accessing
	list head efficiently.
	(tzdb_list::_Node::_S_cache_list_head): New function for
	updating _S_list_head.