libstdc++: Use __is_single_threaded in locale initialization

libstdc++: Use __is_single_threaded in locale initialization

This replaces a __gthread_active_p() check with __is_single_threaded()
so that std::locale initialization doesn't use __gthread_once if it
happens before the first thread is created.

This means that _S_initialize_once() might now be called twice instead
of only once, because if __is_single_threaded() changes to false then we
will do the __gthread_once call even if _S_initialize_once() was already
called. Add a check to _S_initialize_once() and return immediately if
it is the second call.

Also use __builtin_expect to _S_initialize, as the branch will be taken
at most once in the lifetime of the program.

libstdc++-v3/ChangeLog:

	* src/c++98/locale_init.cc (_S_initialize_once): Check if
	initialization has already been done.
	(_S_initialize): Replace __gthread_active_p with
	__is_single_threaded. Use __builtin_expect.