Warn when compiled functions contain hard-coded pointers

[brenhinkeller]

This should probably be a warning rather than a hard error for now, but it would be very useful to know, and I think (?) it may be possible to check for this using LLVM?

[brenhinkeller]

Inspired by #101

[MasonProtter]

I'd also like to just straight up throw an error if I find things like jl_throw_generic in the LLVM for standalone compiled code.

[brenhinkeller]

Looks like this was implemented in #146!