Fix random INT4 and INT8 dtype ranges and clamp attrs

Change range of INT4 to -8 to 7 and INT8 to -128 to 127.
Change clamp attributes to be in range of signed input type.

Change-Id: Iccee1bf29b12da6f7622922f89d59cc0730460c2
Signed-off-by: Jeremy Johnson <jeremy.johnson@arm.com>