base: Move AtomicOpFunctors to a dedicated header

base: Move AtomicOpFunctors to a dedicated header

src/base/types.hh file definition is:

/**
 * @file
 * Defines global host-dependent types:
 * Counter, Tick, and (indirectly) {int,uint}{8,16,32,64}_t.
 */

I feel AtomicOpFunctor doesn't fall in this cathegory so I am
moving those into a dedicated header: base/amo.hh

Change-Id: I8f05fb0944c03e4053cfaf2ffe65cac803df1d93
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23563
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>