Custom pre/postconditions for llvm_ffi_setup

[qsctr]

Provide the ability to specify custom pre/postconditions for the LLVM setup generated by llvm_ffi_setup.

Possible approaches are:

• Have a version of llvm_ffi_setup that outputs as text the setup script that it generates. Then the user can manually edit it to add any extra things they need. However this may result in less maintainable code (like the problem described in Refactor skeleton / boilerplate generation #668). Note: this functionality may be useful in general outside of this use case, just to see what exactly the command is generating. However this is complicated by the fact that currently the command is just directly running the underlying Haskell computations for setting up crucible (that is, the LLVMSetup value that it generates represents just a Haskell computation and not a piece of SAWScript source code), so we would need to generate the SAWScript code separately or abstract it enough that it can do both.
• Have a way to pass additional arguments to llvm_ffi_setup that specify pre/postconditions.
• Decompose llvm_ffi_setup into multiple steps which are sequenced together and which allow specifying pre/postconditions in between, similar to the solution in Refactor skeleton / boilerplate generation #668.

Personally I feel like the second and third options are nicer than the first, but we would have to think about how exactly to define the interface.