sim: Use the [[deprecated()]] attribute instead of GEM5_DEPRECATED.

The [[deprecated()]] attribute is now standard c++, so use that
directly.

Change-Id: I246551b05484a707ac9da05dc91af34d78aae1ff
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48507
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/sim/aux_vector.hh

@@ -131,8 +131,8 @@ enum AuxiliaryVectorType
 #undef GEM5_DEPRECATE_AT
 
 template <class IntType>
-using AuxVector GEM5_DEPRECATED(
-        "The AuxVector template is now in the gem5::auxv namespace.") =
+using AuxVector [[deprecated(
+        "The AuxVector template is now in the gem5::auxv namespace.")]] =
         gem5::auxv::AuxVector<IntType>;
 
 } // namespace gem5