libstdc++: Fix regression in std::list::sort [PR66742]

The standard does not require const-correct comparisons in list::sort.

libstdc++-v3/ChangeLog:

	PR libstdc++/66742
	* include/bits/list.tcc (list::sort): Use mutable iterators for
	comparisons.
	* include/bits/stl_list.h (_Scratch_list::_Ptr_cmp): Likewise.
	* testsuite/23_containers/list/operations/66742.cc: Check
	non-const comparisons.