libstdc++: Optimize std::bitset<N>::to_string

libstdc++: Optimize std::bitset<N>::to_string

This makes to_string approximately twice as fast at any optimization
level. Instead of iterating through every bit, jump straight to the next
bit that is set, by using _Find_first and _Find_next.

libstdc++-v3/ChangeLog:

	* include/std/bitset (bitset::_M_copy_to_string): Find set bits
	instead of iterating over individual bits.