libstdc++: Use unsigned division in std::rotate [PR113811]

Signed 64-bit division is much slower than unsigned, so cast the n and
k values to unsigned before doing n %= k. We know this is safe because
neither value can be negative.

libstdc++-v3/ChangeLog:

	PR libstdc++/113811
	* include/bits/stl_algo.h (__rotate): Use unsigned values for
	division.