Use gmtime’s target

Use gmtime’s target

gmtime has a target allocated for it, but it hasn’t used that target
since 46fc3d4c6 (inseparable changes from match from perl-5.003_97g to
perl-5.003_97h, probably the ‘Mondo Cool patch’ mentioned in the com-
mit message).

There is no reason not to use it and avoid allocating a fresh SV with
each scalar-context call.