libstdc++: Use __libc_single_threaded for locale initialization

Most initialization of locales and facets happens before main() during
startup, when the program is likely to only have one thread. By using
the new __gnu_cxx::__is_single_threaded() function instead of checking
__gthread_active_p() we can avoid using pthread_once or atomics for the
common case.

That said, I'm not sure why we don't just use a local static variable
instead, as __cxa_guard_acquire() already optimizes for the
single-threaded case:

  static const bool init = (_S_initialize_once(), true);

I'll revisit that for GCC 12.

libstdc++-v3/ChangeLog:

	* src/c++98/locale.cc (locale::facet::_S_get_c_locale())
	(locale::id::_M_id() const): Use __is_single_threaded.
	* src/c++98/locale_init.cc (locale::_S_initialize()):
	Likewise.

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