libstdc++: Partially revert r11-5314

libstdc++: Partially revert r11-5314

The changes in r11-5314 are broken, because it means we don't use
__gthread_once for the first few initializations, but after the program
becomes multi-threaded we will repeat the initialization, using
__gthread_once once this time. This leads to memory errors.

The use of __is_single_threaded() in locale::id::_M_id() is OK, because
the side effects are the same either way.

libstdc++-v3/ChangeLog:

	* src/c++98/locale.cc (locale::facet::_S_get_c_locale()):
	Revert change to use __is_single_threaded.
	* src/c++98/locale_init.cc (locale::_S_initialize()):
	Likewise.