[Perl/perl5] 11c545: XSUB.h: use Stack_off_t for AX and items

[Tony Cook via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: 11c545feeab1b822b7ebf4fca92645a4bfcf49b5
      
https://github.com/Perl/perl5/commit/11c545feeab1b822b7ebf4fca92645a4bfcf49b5
  Author: Tony Cook <t...@develop-help.com>
  Date:   2024-01-11 (Thu, 11 Jan 2024)

  Changed paths:
    M XSUB.h
    M embed.fnc
    M proto.h
    M util.c

  Log Message:
  -----------
  XSUB.h: use Stack_off_t for AX and items

I hadn't expected code to be taking pointers or references to AX,
which turned out to be wrong, so make them Stack_off_t.

This allows XS::Framework or similar code to build with a default
build of perl, but it will still fail to build if perl is built
with -DPERL_STACK_OFFSET_SSIZET, which can only be fixed by updating
XS::Framework to use Stack_off_t itself.

Fixes #21782