libstdc++: Fix std::span constraint for sizeof(size_t) < sizeof(int) [PR108221]

libstdc++: Fix std::span constraint for sizeof(size_t) < sizeof(int) [PR108221]

The default constructor has a constraint that is always false if
arithmetic on size_t values promotes to int. Rewrite the constraint
exactly as written in the standard, which works correctly.

libstdc++-v3/ChangeLog:

	PR libstdc++/108221
	* include/std/span (span::span()): Un-simplify constraint to
	work for size_t of lesser rank than int.