stdlib: Fix __getrandom_nocancel type and arc4random usage (BZ #29638)

Using an unsigned type prevents the fallback to be used if kernel
does not support getrandom syscall.

Checked on x86_64-linux-gnu.

Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>

stdlib/arc4random.c

@@ -34,7 +34,7 @@ void
 __arc4random_buf (void *p, size_t n)
 {
   static int seen_initialized;
-  size_t l;
+  ssize_t l;
   int fd;
 
   if (n == 0)