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>
(cherry picked from commit 13db9ee2)