The old formulation (with system calls as block terminators) proved to be
impossible to implement properly. Handlers for syntax overrides have very
limited types (`IO`, rather than `OverrideSim`), which made symbolic branching
and reusing overrides impossible.
This change replaces the system call block terminator with an arch-specific
function that is translated into a function handle lookup (which is then
dispatched to with a call).
Unfortunately, this refactoring required combining the AArch32 simplification
module with the architecture extension definitions, due to the new translation
relying on the simplifier instance.
