[Perl/perl5] 75da90: cop.h: Extend core-only portion

[Karl Williamson via perl5-changes]

  Branch: refs/heads/smoke-me/khw-pr4
  Home:   https://github.com/Perl/perl5
  Commit: 75da90dc9e08341e8e96b120c64e4c7bc007a66a
      
https://github.com/Perl/perl5/commit/75da90dc9e08341e8e96b120c64e4c7bc007a66a
  Author: Karl Williamson <k...@cpan.org>
  Date:   2020-11-22 (Sun, 22 Nov 2020)

  Changed paths:
    M cop.h

  Log Message:
  -----------
  cop.h: Extend core-only portion

This encloses some #defines in a PERL_CORE section, as their only use is
in the macro immediately following, already confined to core.