arch-riscv,tests: small update to make gem5.fast compile

arch-riscv,tests: small update to make gem5.fast compile

This small change is meant to enable gem5.fast compilation
for riscv. Also, the riscv tests based on gem5.fast
work now and are removed from .testignore.

Change-Id: Id9c96d8e4682a74ec1d77a66eae53d6f9b64d302
Issue-On: https://gem5.atlassian.net/browse/GEM5-526
Issue-On: https://gem5.atlassian.net/browse/GEM5-527
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28893
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>