Make XS sort routines work again

Make XS sort routines work again

These stopped working when the CvROOT and CvXSUB fields were merged
in 5.10.0:

$ perl5.8.9 -le 'print sort utf8::is_utf8 2,1'
Usage: utf8::is_utf8(sv) at -e line 1.
$ perl5.10.0 -le 'print sort utf8::is_utf8 2,1'
12

(In the latter case, the utf8::is_utf8 routine is not being called.)

pp_sort has this:

	    if (!(cv && CvROOT(cv))) {
		if (cv && CvISXSUB(cv)) {

But CvROOT is the same as CvXSUB, so that block is never entered for
XSUBs, so this piece of code later on:

	    if (is_xsub)
		PL_sortcop = (OP*)cv;
	    else
		PL_sortcop = CvSTART(cv);

sets PL_sortcop to CvSTART for XSUBs, but CvSTART is NULL.  Later on,
this if condition fails:

	if (PL_sortcop) {

so the XSUB is treated as being absent.