arch-arm: Stop "using namespace std"

arch-arm: Stop "using namespace std"

Change-Id: If0f373bdaadce81c5ebbc37b03810335c42dd10a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39561
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>