sv.c: Suppress compiler warning

Cast to U32 explicitly now that arena_size is U32.  See
<https://rt.perl.org/Ticket/Display.html?id=122861#txn-1310985>.
perldelta: Move an entry to the right section

5b306eef3 added it to the wrong section because the patch did not
apply cleanly with git-am and the ‘patch’ program seemed to have
no trouble with it, and I applied it without double-checking what
patch had done.  Trouble with pod documents is that so many parts
of them look the same that things always apply, if wrongly. :-)