base,dev,mem-ruby: Rename m5 namespace as gem5

As part of recent decisions regarding namespace
naming conventions, all namespaces will be changed
to snake case.

::m5 became ::gem5.

Change-Id: I250b4354113fcd6005dc4144ae378552cb8f6717
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45437
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>

src/dev/arm/gic_v3_its.hh

@@ -354,7 +354,7 @@ class ItsProcess : public Packet::SenderState
     using DTE = Gicv3Its::DTE;
     using ITTE = Gicv3Its::ITTE;
     using CTE = Gicv3Its::CTE;
-    using Coroutine = m5::Coroutine<PacketPtr, ItsAction>;
+    using Coroutine = gem5::Coroutine<PacketPtr, ItsAction>;
     using Yield = Coroutine::CallerType;
 
     ItsProcess(Gicv3Its &_its);

src/dev/arm/smmu_v3_proc.hh

@@ -94,7 +94,7 @@ struct SMMUSignal
 class SMMUProcess : public Packet::SenderState
 {
   private:
-    typedef m5::Coroutine<PacketPtr, SMMUAction> Coroutine;
+    typedef gem5::Coroutine<PacketPtr, SMMUAction> Coroutine;
 
     Coroutine *coroutine;
     std::string myName;