arch-riscv: use sext rather than manual masks

arch-riscv: use sext rather than manual masks

Replace manual creation of masks for sign extension of immediates with
the sext<N> function.

Change-Id: Ief2df91a25500c64f5bcae0dcd437c1e3bb95e6c
Reviewed-on: https://gem5-review.googlesource.com/6182
Reviewed-by: Alec Roelke <ar4jc@virginia.edu>
Maintainer: Alec Roelke <ar4jc@virginia.edu>