arm,base,gpu: Use std::make_unique instead of m5::make_unique.

arm,base,gpu: Use std::make_unique instead of m5::make_unique.

Now that we're using c++14, we can just assume that std::make_unique
exists. We no longer have to conditionally inject our own version.

Change-Id: I5d851afb02dd05c7af93864ffec3b3184f3d4ec8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35215
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>