arch-riscv: Stop "using namespace std"

arch-riscv: Stop "using namespace std"

Change-Id: Ia18ac69797019fb853b7c07a3961840ee6b0df39
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39558
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>