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/base/coroutine.test.cc

@@ -39,7 +39,7 @@
 
 #include "base/coroutine.hh"
 
-using namespace m5;
+using namespace gem5;
 
 /**
  * This test is checking if the Coroutine, once it's created