libstdc++: Fix regression in std::stable_sort

libstdc++: Fix regression in std::stable_sort

The recent change to split out the cold path of std::stable_sort caused
a regression for some Qt code. The problem is that the library now adds
a value of type ptrdiff_t to the iterator, which is ambiguous with
-pedantic. The addition could either convert the iterator to a built-in
pointer and add the ptrdiff_t to that, or it could convert the ptrdiff_t
to the iterator's difference_type and use the iterator's own operator+.

The fix is to cast the ptrdiff_t value to the difference type first.

libstdc++-v3/ChangeLog:

	* include/bits/stl_algo.h (__stable_sort): Cast size to
	iterator's difference type.
	* testsuite/25_algorithms/stable_sort/4.cc: New test.