arch: arc: fix the bug of blt in syscall

blt is signed comparsion, if r6 is a negative number created by
malicious code, it will pass the check, bring a secure risk.

use blo (unsinged comparison) to do the check.

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>
Signed-off-by: David Brown <david.brown@linaro.org>