arch,sim,kern,dev,cpu: Create a Workload SimObject.

arch,sim,kern,dev,cpu: Create a Workload SimObject.

This generalized Workload SimObject is not geared towards FS or SE
simulations, although currently it's only used in FS. This gets rid
of the ARM specific highestELIs64 property (from the workload, not the
system) and replaces it with a generic getArch.

The old globally accessible kernel symtab has been replaced with a
symtab accessor which takes a ThreadContext *. The parameter isn't used
for anything for now, but in cases where there might be multiple
symbol tables to choose from (kernel vs. current user space?) the
method will now be able to distinguish which to use. This also makes
it possible for the workload to manage its symbol table with whatever
policy makes sense for it.

That method returns a const SymbolTable * since most of the time the
symbol table doesn't need to be modified. In the one case where an
external entity needs to modify the table, two pseudo instructions,
the table to modify isn't necessarily the one that's currently active.
For instance, the pseudo instruction will likely execute in user space,
but might be intended to add a symbol to the kernel in case something
like a module was loaded.

To support that usage, the workload has a generic "insertSymbol" method
which will insert the symbol in the table that "makes sense". There is
a lot of ambiguity what that means, but it's no less ambiguous than
today where we're only saved by the fact that there is generally only
one active symbol table to worry about.

This change also introduces a KernelWorkload SimObject class which
inherits from Workload and adds in kernel related members for cases
where the kernel is specified in the config and loaded by gem5 itself.
That's the common case, but the base Workload class would be used
directly when, for instance, doing a baremetal simulation or if the
kernel is loaded by software within the simulation as is the case for
SPARC FS.

Because a given architecture specific workload class needs to inherit
from either Workload or KernelWorkload, this change removes the
ability to boot ARM without a kernel. This ability should be restored
in the future.

To make having or not having a kernel more flexible, the kernel
specific members of the KernelWorkload should be factored out into
their own object which can then be attached to a workload through a
(potentially unused) property rather than inheritance.

Change-Id: Idf72615260266d7b4478d20d4035ed5a1e7aa241
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24283
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>