gpu-compute: Handle mailbox/wakeup signals for GPUFS
gpu-compute: Handle mailbox/wakeup signals for GPUFS

The current mailbox/wakeup signal uses the SE mode proxy port to write
the event value. This is not available in full system mode so instead we
need to issue a DMA write to the address. The value of event_val clears
the event.

Change-Id: I424469076e87e690ab0bb722bac4c3e7414fb150
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/57709
Reviewed-by: Matt Sinclair <d2406197fb1bedd1e23c70c50c510ff0e9ab0f5b@gmail.com>
Maintainer: Matt Sinclair <d2406197fb1bedd1e23c70c50c510ff0e9ab0f5b@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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