stdlib: Incorporating multi-isa work to the stdlib

The main restriction with this design is it results in one ISA target
per board. The ISA is declared per core. To make the design simpler it's
assumed a Processor (a collection of cores) are all of the same ISA. As
each board has one processor, this also means a board is typically tied
to one ISA per simulation.

In order to remain backwards compatible and maintain the standard
library APIs, this patch adds a `--main-isa` parameter which will
determine what `gem5.runtime.get_runtime_isa` returns in cases where
mutliple ISAs are compiled in. When setting the ISA in a simulation (via
the Processor or Cores), the user may, as before, choose not to and, in
this case, the `gem5.runtime.get_runtime_isa` function is used.

The `gem5.runtime.get_runtime_isa` function is an intermediate step
which should be removed in future versions of gem5 (users should specify
precisely what ISA they want via configuration scripts). For this reason
it throws a warning when used and should not be heavily relied upon. It
is deprecated.

Change-Id: Ia76541bfa9a5a4b6b86401309281849b49dc724b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/55423
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/python/m5/main.py

@@ -107,6 +107,12 @@ def parse_options():
         help="Reduce verbosity")
     option('-v', "--verbose", action="count", default=0,
         help="Increase verbosity")
+    option('--main-isa', action='store', default=None,
+          help='Select the main target ISA. This dictates what the '
+          '`gem5.runtime.get_runtime_isa()` function returns when multiple '
+          'ISAs are compiled into the gem5 binary. Note: This functionality '
+          'is deprecated. The `gem5.runtime.get_runtime_isa()` function will '
+          'be removed in future releases of gem5.')
 
     # Statistics options
     group("Statistics Options")