Revert "PERL_SET_LOCALE_CONTEXT: Actually do something"

This reverts commit 9e254b0.
Date:   Wed Apr 5 12:26:26 2023 -0600

This fixes GH #21040

The reverted commit caused failures in platforms using the musl library,
notably Alpine Linux.  I came up with a fix for that, which instead
broke Windows.  In looking at that I realized the original fix is
incomplete, and that things are too precarious to try to fix so close to
5.38.0.  For example, I spent hours, due to a %p format printing 0 for
what turned out to be a non-NULL string pointer.  I think it has to do
do with the fact that the failing code is in the middle of transitioning
between threads, and the printing got confused as a result.

The reverted commit was part of a series fixing #20155 and #20231.  But
the earlier part of the series succeeded in fixing those, without that
commit, so reverting it should not cause things to break as a result.

This whole issue has to do with locales and threading.  Those still
don't play well together.  I have a series of well over 200 commits that
address this situation, for applying in early 5.39.  My point is that we
are a long way from solving these kinds of issues; and they don't come
up that much in the field because they just don't get used.  The
reverted commit would help if it worked properly, but it's not the only
thing wrong by a long shot.

perl.h

@@ -6365,7 +6365,7 @@ EXTCONST U8   PL_deBruijn_bitpos_tab64[];
 #ifdef USE_PERL_SWITCH_LOCALE_CONTEXT
 #  define PERL_SET_LOCALE_CONTEXT(i)                                        \
       STMT_START {                                                          \
-          if (LIKELY(! PL_veto_switch_non_tTHX_context))                    \
+          if (UNLIKELY(PL_veto_switch_non_tTHX_context))                    \
                 Perl_switch_locale_context();                               \
       } STMT_END
 #else