XXX experimental: Define setlocale_i() on unsafe threaded builds

On threaded Configurations where thread-safe locale handling is not
available, perl automatically does a modicum of prevention of races by
executing locale changes in a critical section, and copying the results
to a thread-safe location.

This commit defines setlocale_i() on such builds.  This macro is used to
bypass more complex handling required in fully thread-safe builds.

I don't see how this changes thread-safeness on the unsafe builds, and
it passes the tests, but I'm considering it experimental for now, until
later in the development cycle.

locale.c

@@ -2180,6 +2180,14 @@ S_less_dicey_bool_setlocale_r(pTHX_ const int cat, const char * locale)
 #  define void_setlocale_c(cat, locale) void_setlocale_r(cat, locale)
 #  define void_setlocale_i(i, locale)   void_setlocale_r(categories[i], locale)
 
+/*---------------------------------------------------------------------------*/
+
+/* XXX unclear if should define this
+ * setlocale_i is only defined for Configurations where the libc setlocale()
+ * doesn't need any further tweaking (beyond that just above).  It allows for
+ * some shortcuts */
+#  define setlocale_i(i, locale)  less_dicey_setlocale_r(categories[i], locale)
+
 /*===========================================================================*/
 
 #elif defined(USE_POSIX_2008_LOCALE)