arch,sim: Rename GuestABI namespace as guest_abi

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

::GuestABI became ::guest_abi.

Change-Id: I68700ef63479f1bb3eeab044b29dc09d86424608
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45433
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Maintainer: Gabe Black <gabe.black@gmail.com>

src/sim/proxy_ptr.hh

@@ -354,7 +354,8 @@ operator + (A a, const ProxyPtr<T, Proxy> &other)
     return other + a;
 }
 
-namespace GuestABI
+GEM5_DEPRECATED_NAMESPACE(GuestABI, guest_abi);
+namespace guest_abi
 {
 
 template <typename ABI, typename T, typename Proxy>
@@ -379,7 +380,7 @@ struct Argument<ABI, ConstProxyPtr<T, Proxy>>
     }
 };
 
-} // namespace GuestABI
+} // namespace guest_abi
 
 template <typename T, typename Proxy>
 std::ostream &