stdlib: fix arc4random fallback to /dev/urandom (BZ 31612)

The __getrandom_nocancel used by __arc4random_buf uses
INLINE_SYSCALL_CALL (which returns -1/errno) and the loop checks for
the return value instead of errno to fallback to /dev/urandom.

The malloc code now uses __getrandom_nocancel_nostatus, which uses
INTERNAL_SYSCALL_CALL, so there is no need to use the variant that does
not set errno (BZ#29624).

Checked on x86_64-linux-gnu.

Reviewed-by: Xi Ruoyao <xry111@xry111.site>

stdlib/arc4random.c

@@ -51,7 +51,7 @@ __arc4random_buf (void *p, size_t n)
 	  n -= l;
 	  continue; /* Interrupted by a signal; keep going.  */
 	}
-      else if (l == -ENOSYS)
+      else if (l < 0 && errno == ENOSYS)
 	break; /* No syscall, so fallback to /dev/urandom.  */
       arc4random_getrandom_failure ();
     }