[Perl/perl5] 37f50f: Add setlocale_i()

[Karl Williamson via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: 37f50f0f03b7d6b4d6a60cae76b00a4f87308929
      
https://github.com/Perl/perl5/commit/37f50f0f03b7d6b4d6a60cae76b00a4f87308929
  Author: Karl Williamson <k...@cpan.org>
  Date:   2023-09-13 (Wed, 13 Sep 2023)

  Changed paths:
    M locale.c

  Log Message:
  -----------
  Add setlocale_i()

This macro is defined on Configurations where a simple unadorned
setlocale works for categories that don't have perl record keeping
involved.

This allows Perl_setlocale() to avoid a lot of unnecessary overhead for
these.

This commit defines it only for unthreaded perls.