[libc] Fix erroneous warning on GCC (llvm#101520)

Author: jhuber6

libc/src/stdio/printf_core/writer.h

@@ -100,6 +100,7 @@ struct WriteBuffer {
     case WriteMode::RESIZE_AND_FILL_BUFF:
       return resize_and_write(new_str);
     }
+    __builtin_unreachable();
   }
 };