x86: De-x86ify the IntMasterPort.

x86: De-x86ify the IntMasterPort.

The devices which host an IntMasterPort are very specific to x86 at the
moment, but the ports don't have to be. This change moves
responsibilities around so that the x86 specific aspects are handled
in the device, and the ports themselves are ISA agnostic.

Change-Id: I50141b66895be7d8f6303605505002ef424af7fd
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20827
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>