Cygwin: fix LC_CTYPE in global locale to be a real C.UTF-8 locale

https://cygwin.com/pipermail/cygwin/2022-December/252571.html

Cygwin's default locale is "C.UTF-8" as far as LC_CTYPE settings
are concerned.  However, while __global_locale contains fixed
mbtowc and wctomb pointers, the lc_ctype_T pointer is still pointing
to _C_ctype_locale, representing the standard "C" locale.

The problem with this is that the codeset name as well as MB_CUR_MAX
is wrong.

Fix this by introducing a new lc_ctype_T structure called
_C_utf8_ctype_locale, setting the default codeset to "UTF-8" and
MB_CUR_MAX to 6.  Use this as lc_ctype_T pointer in __global_locale
by default on Cygwin.

Fixes: a6a477fa81 ("POSIX-1.2008 per-thread locales, groundwork part 1")
Co-Authored-By: Takashi Yano <takashi.yano@nifty.ne.jp>
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

newlib/libc/locale/lctype.c

@@ -39,6 +39,22 @@ const struct lc_ctype_T _C_ctype_locale = {
 #endif
 };
 
+#ifdef __CYGWIN__
+static char	numsix[] = { '\6', '\0'};
+
+const struct lc_ctype_T _C_utf8_ctype_locale = {
+	"UTF-8",			/* codeset */
+	numsix				/* mb_cur_max */
+#ifdef __HAVE_LOCALE_INFO_EXTENDED__
+	,
+	{ "0", "1", "2", "3", "4",	/* outdigits */
+	  "5", "6", "7", "8", "9" },
+	{ L"0", L"1", L"2", L"3", L"4",	/* woutdigits */
+	  L"5", L"6", L"7", L"8", L"9" }
+#endif
+};
+#endif
+
 /* NULL locale indicates global locale (called from setlocale) */
 int
 __ctype_load_locale (struct __locale_t *locale, const char *name,

newlib/libc/locale/locale.c

@@ -272,10 +272,11 @@ struct __locale_t __global_locale =
     { NULL, NULL },			/* LC_ALL */
 #ifdef __CYGWIN__
     { &_C_collate_locale, NULL },	/* LC_COLLATE */
+    { &_C_utf8_ctype_locale, NULL },	/* LC_CTYPE */
 #else
     { NULL, NULL },			/* LC_COLLATE */
-#endif
     { &_C_ctype_locale, NULL },		/* LC_CTYPE */
+#endif
     { &_C_monetary_locale, NULL },	/* LC_MONETARY */
     { &_C_numeric_locale, NULL },	/* LC_NUMERIC */
     { &_C_time_locale, NULL },		/* LC_TIME */

newlib/libc/locale/setlocale.h

@@ -64,6 +64,9 @@ struct lc_ctype_T
 #endif
 };
 extern const struct lc_ctype_T _C_ctype_locale;
+#ifdef __CYGWIN__
+extern const struct lc_ctype_T _C_utf8_ctype_locale;
+#endif
 
 struct lc_monetary_T
 {