libstdc++: Optimize ref-count updates in COW std::string

Most ref-count updates in the COW string are done via the functions in
<ext/atomicity.h>, which will use non-atomic ops when the program is
known to be single-threaded. The _M_is_leaked() and _M_is_shared()
functions use __atomic_load_n directly, because <ext/atomicity.h>
doesn't provide a load operation. Those functions can check the
__is_single_threaded() predicate to avoid using __atomic_load_n when not
needed.

The move constructor for the fully-dynamic-string increments the
ref-count by either 2 or 1, for leaked or non-leaked strings
respectively. That can be changed to use a non-atomic store of 1 for all
non-shared strings. It can be non-atomic because even if the program is
multi-threaded, conflicting access to the rvalue object while it's being
moved from would be data race anyway. It can store 1 directly for all
non-shared strings because it doesn't matter whether the initial
refcount was -1 or 0, it should be 1 after the move constructor creates
a second owner.

libstdc++-v3/ChangeLog:

	* include/bits/cow_string.h (basic_string::_M_is_leaked): Use
	non-atomic load when __is_single_threaded() is true.
	(basic_string::_M_is_shared): Likewise.
	(basic_string::(basic_string&&)) [_GLIBCXX_FULLY_DYNAMIC_STRING]:
	Use non-atomic store when rvalue is not shared.

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