libstdc++: Fix std::lexicographic_compare for unsigned char (PR 93972)

libstdc++: Fix std::lexicographic_compare for unsigned char (PR 93972)

The new 25_algorithms/lexicographical_compare/93972.cc test fails on
targets where char is unsigned, revealing an existing regression with
the std::__memcmp helper that had gone unnoticed in
std::lexicographical_compare. When comparing char and unsigned char, the
memcmp optimisation is enabled, but the new std::__memcmp function fails
to compile for mismatched types.

	PR libstdc++/93972
	* include/bits/stl_algobase.h (__memcmp): Allow pointer types to
	differ.
	* testsuite/25_algorithms/lexicographical_compare/uchar.cc: New test.