libstdc++: Fix src/c++20/tzdb.cc for non-constexpr std::mutex

Building libstdc++ reportedly fails for targets without lock-free
std::atomic<T*> which don't define __GTHREAD_MUTEX_INIT:

src/c++20/tzdb.cc:110:21: error: 'constinit' variable 'std::chrono::{anonymous}::list_mutex' does not have a constant initializer
src/c++20/tzdb.cc:110:21: error: call to non-'constexpr' function 'std::mutex::mutex()'

The solution implemented by this commit is to use a local static mutex
when it can't be constinit, so that it's constructed on first use.

With this change, we can also simplify the preprocessor logic for
defining USE_ATOMIC_SHARED_PTR. It now depends on the same conditions as
USE_ATOMIC_LIST_HEAD, so in theory we could have a single macro. Keeping
them separate would allow us to replace the use of atomic<shared_ptr<T>>
with a mutex if that performs better, without having to give up on the
lock-free cache for fast access to the list head.

libstdc++-v3/ChangeLog:

	* src/c++20/tzdb.cc (USE_ATOMIC_SHARED_PTR): Define consistently
	with USE_ATOMIC_LIST_HEAD.
	(list_mutex): Replace global object with function. Use local
	static object when std::mutex constructor isn't constexpr.

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