misc,ext: Add gem5/SST integration

Enable a gem5 system to be an SST::Component.

This change includes,
  - SST::gem5Component: responsible for,
      - initializing the Python environment for gem5
      - instantiating gem5 SimObjects
      - connecting SST::SSTResponderSubComponent to
      - hanlding the gem5 event queue (no thread-synchronization)
      - handling executing gem5 events
  - SST::SSTResponderSubComponent: responsible for,
      - receiving gem5 requests and sending the requests to
      - sending responses to the corresponding
  - SST::SSTResponder: owned by SSTResponderSubComponent, the
actual actor that sends gem5's requests to memory.
  - gem5::OutgoingRequestBridge: receives the requests from
gem5 and sends them to SST. This SimObject allows the initialization
requests to be cached and the receiver could query the
initialization data later on.
  - gem5::SSTResponderInterface: the interface specifying how SST
communicates with gem5.
  - A working example of a gem5/SST setup.

More information is available at ext/sst/README.md.
For installation instructions, please refer to ext/sst/INSTALL.md.

Change-Id: I6b81260ef825415bcfe72b8a078854f4c94de782
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50468
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
28 files changed