dev-hsa,gpu-compute: Agent Packet handler implemented.

dev-hsa,gpu-compute: Agent Packet handler implemented.

HSA packet processor will now accept and process agent packets.

Type field in packet is command type.
For now:
        AgentCmd::Nop = 0
        AgentCmd::Steal = 1

Steal command steals the completion signal for a running kernel.
This enables a benchmark to use hsa primitives to send an agent
packet to steal the signal, then wait on that signal.

Minimal working example to be added in gem5-resources.

Change-Id: I37f8a4b7ea1780b471559aecbf4af1050353b0b1
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/37015
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>