hppa: Fix decrement_and_branch_until_zero constraint

The third alternative for argument 4 needs to be an early clobber
constraint.  Noticed testing LRA.

2024-11-12  John David Anglin  <danglin@gcc.gnu.org>

gcc/ChangeLog:

	* config/pa/pa.md (decrement_and_branch_until_zero): Fix
	constraint.