Support Cross Compilation Targets

[2over12]

For crates that only build on certain platforms it is convenient to use a cross compilation target. Setting up the default target almost works but a target specific dir would need to be pushed here: https://github.com/trailofbits/siderophile/blob/debb857f97d5fa71b89b9cf9d103c25371b42b23/src/callgraph_gen.rs#L109

Probably the easiest way to do this would be to allow passing down a target flag to cargo. If the target flag gets passed down, then the bitcode gets looked for in a non-default dir.

[2over12]

I've hacked around this for now but if we think this is a good feature I can probably have something done pretty quickly