libstdc++/112351 - deal with __gthread_once failure during locale init

The following makes the C++98 locale init path follow the way the
C++11 performs initialization.  This way we deal with pthread_once
failing, falling back to non-threadsafe initialization which, given we
initialize from the library, should be serialized by the dynamic
loader already.

	PR libstdc++/112351
libstdc++-v3/
	* src/c++98/locale.cc (locale::facet::_S_initialize_once):
	Check whether _S_c_locale is already initialized.
	(locale::facet::_S_get_c_locale): Always perform non-threadsafe
	init when threadsafe init failed.