locale.c: Skip locale utf8ness calculation if feasible

I originally wrote this to save time processing the strings returned by
localeconv().  If we know that the locale isn't UTF-8, then we don't
have to calculate this for each string returned, Thus "Calculate once, not
many times".

But I hadn't realized that likely only one string is ever going to be
non-ASCII: the currency symbol.  (The decimal and thousands separators
could also be, but of the >500 locales on my Linux box, only ps_AF has
them so.  That is the Pashtun language in Afghanistan; not a frequently
occurring locale.)

So it comes down to either case is effectively "calculate once".  And it
is generally more expensive to calculate the UTF8ness of a locale than a
particular string in it, especially when those strings are going to be
ASCII, as in this case.

This commit changes to not calculate the locale's UTF8ness.

locale.c

@@ -5722,11 +5722,6 @@ S_my_localeconv(pTHX_ const int item)
 
     for (unsigned int i = 0; i < 2; i++) {  /* Try both types of strings */
 
-        const char * locale = locales[i];
-        if (! is_locale_utf8(locale)) {
-            continue;   /* No string can be UTF-8 if the locale isn't */
-        }
-
         /* Examine each string */
         for (const lconv_offset_t *strp = strings[i]; strp->name; strp++) {
             const char * name = strp->name;
@@ -5740,8 +5735,8 @@ S_my_localeconv(pTHX_ const int item)
 
             /* Determine if the string should be marked as UTF-8. */
             if (UTF8NESS_YES == (get_locale_string_utf8ness_i(SvPVX(*value),
-                                                  LOCALE_IS_UTF8,
-                                                  NULL,
+                                                  LOCALE_UTF8NESS_UNKNOWN,
+                                                  locales[i],
                                                   LC_ALL_INDEX_ /* OOB */)))
             {
                 SvUTF8_on(*value);