scons: Generalize building binaries.

Building gem5 binaries or regression test binaries needs to be done
from within the make_env function which builds an environment for each
flavor of build (opt, fast, debug, etc.). That makes it impossible to
add new types of binaries without modifying the central SConscript.

This change refactors how binaries are set up so that the class that
represents them handles the details of how the binary should be built.
Also, a metaclass and some lists track types of binaries and individual
instances of binaries so that they can be iterated over automatically
in make_env.

Each new executable class can define a declare_all class function which
calls declare() on individual instances. declare_all is a place to do
any processing that only has to happen once (for instance specializing
the environment) for a particular family of executables.

Change-Id: I8a6ee9438280cd67e6c0b92ca28738a53cb16950
Reviewed-on: https://gem5-review.googlesource.com/10915
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

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