Make &xsub and goto &xsub work with tied @_

Make &xsub and goto &xsub work with tied @_

This is the only place where tied @_ does not work, and there appears
to be no reason why it shouldn’t, apart from the fact that it hasn’t
been implemented.

Commit 67955e0c was what made &xsub work to begin with.  93965878572
introduced tied arrays and added the comment to pp_entersub saying
that @_ is not tiable.

goto &xsub has worked since perl 5.000, but 93965878572 did not make
it work with tied arrays.