fastmodel: Add glue code which adapts fastmodels to run in gem5.

fastmodel: Add glue code which adapts fastmodels to run in gem5.

Fast Models are models written by ARM which emulate different
components of a computer system. They can be combined into small
subsystems and then exported as systemc modules.

To enable this code, you'll need to set USE_ARM_FASTMODEL variable to
true. This CL does not include the fast models themselves, or a license
to use them or the associated tools. To build these fast models, you'll
need to set some scons variables. These variables should be set as
described in the fast model distribution.

* PVLIB_HOME
* MAXCORE_HOME
* ARMLMD_LICENSE_FILE

Some minor patches to source filesdistributed with the fast model code
may be necessary since their use of systemc is not necessarily 100%
standards compliant.

Change-Id: Id53814b95d8aa320da4d4f2159be0736fc12eb73
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20799
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>