libstdc++: Restore unconditional atomic load in COW std::string

The relaxed load is already optimal, checking the __single_threaded
global before doing a non-atomic load isn't an optimization.

libstdc++-v3/ChangeLog:

	* include/bits/cow_string.h (basic_string::_M_is_leaked()):
	Revert change to check __is_single_threaded() before using
	atomic load.