[Perl/perl5] a21a2d: pp.c: Use utf8_hop_back instead of rolling our own

[Karl Williamson via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: a21a2dda119c764ed17277d02fcc79e8e7a0a025
      
https://github.com/Perl/perl5/commit/a21a2dda119c764ed17277d02fcc79e8e7a0a025
  Author: Karl Williamson <k...@cpan.org>
  Date:   2023-07-17 (Mon, 17 Jul 2023)

  Changed paths:
    M pp.c

  Log Message:
  -----------
  pp.c: Use utf8_hop_back instead of rolling our own

This is inlined, so shouldn't be any slower, and fixing bugs will be
done in a single place