riscv: fixed syscall return value

riscv: fixed syscall return value

In case of failure, a syscall returns a negative value encoding the
error code. This patch makes the risc-v implementation returns the
encoded value instead of its absolute value upon a failure of a syscall.

Change-Id: I6032b0337fe1cff5b326dbc6bb3b87a415f03300
Reviewed-on: https://gem5-review.googlesource.com/c/9627
Reviewed-by: Alec Roelke <ar4jc@virginia.edu>
Maintainer: Alec Roelke <ar4jc@virginia.edu>