Only use "using namespace" with the *ISAInst namespace, not the top
level namespace. Also only using namespace *ISA, and not the gem5
namespace itself. The *ISAInst namespace is already in the gem5
namespace, and so will resolve names in it automatically.
Change-Id: Iebf3c9519c65baba073d73744665f8c98880804c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49887
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
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