microblaze: add 'fallthrough' to memcpy/memset/memmove

Fix "fallthrough" warnings in microblaze memcpy/memset/memmove
library functions.

  CC      arch/microblaze/lib/memcpy.o
../arch/microblaze/lib/memcpy.c: In function 'memcpy':
../arch/microblaze/lib/memcpy.c:70:4: warning: this statement may fall through [-Wimplicit-fallthrough=]
   70 |    --c;
../arch/microblaze/lib/memcpy.c:71:3: note: here
   71 |   case 2:
../arch/microblaze/lib/memcpy.c:73:4: warning: this statement may fall through [-Wimplicit-fallthrough=]
   73 |    --c;
../arch/microblaze/lib/memcpy.c:74:3: note: here
   74 |   case 3:
../arch/microblaze/lib/memcpy.c:178:10: warning: this statement may fall through [-Wimplicit-fallthrough=]
  178 |   *dst++ = *src++;
../arch/microblaze/lib/memcpy.c:179:2: note: here
  179 |  case 2:
../arch/microblaze/lib/memcpy.c:180:10: warning: this statement may fall through [-Wimplicit-fallthrough=]
  180 |   *dst++ = *src++;
../arch/microblaze/lib/memcpy.c:181:2: note: here
  181 |  case 1:
  CC      ...