libgcc: cris: config/cris/arit.c (DS): Apply attribute __fallthrough__.

	* config/cris/arit.c (DS): Apply attribute fallthrough.

Without this, there are, for each compilation of arit.c, 30ish
occurrences of "this statement may fall through
[-Wimplicit-fallthrough=]", for lines that look like
    case 32: DS; case 31: DS; case 30: DS; case 29: DS;

libgcc/ChangeLog

+2020-01-18  Hans-Peter Nilsson  <hp@axis.com>
+
+	* config/cris/arit.c (DS): Apply attribute __fallthrough__.
+
 2020-01-18  John David Anglin  <danglin@gcc.gnu.org>
 
 	PR libgcc/92988

libgcc/config/cris/arit.c

@@ -128,7 +128,8 @@ do_31div (unsigned long a, unsigned long b)
      i.e. "a - (b - 1) == (a - b) + 1".  */
   b--;
 
-#define DS __asm__ ("dstep %2,%0" : "=r" (a) : "0" (a), "r" (b))
+#define DS __asm__ ("dstep %2,%0" : "=r" (a) : "0" (a), "r" (b)); \
+ __attribute__ ((__fallthrough__))
 
   switch (quot_digits)
     {