use @main for juliac executable entry point

[JeffBezanson]

juliac clearly should use the at-main mechanism for executables. That also allows us to call julia_init from C main instead of using __attribute__((constructor)). I also added support for passing in the command line arguments (kind of useful!)

I changed Core.ARGS from a Vector{Any} to a Vector{String}. I don't know why we didn't do that years ago. Surely this is ok?

The biggest problem is looking up julia main from C main. For now I put in a hack to keep the binding around, but it's not very satisfying. Maybe we should insert a ccallable with some known name like juliac_exe_main? Any other ideas on how to handle this? Edit: done.

[fingolfin]

This sounds sensible to me... any suggestions for someone else who would be better-suited than me for reviewing this?

[JeffBezanson]

Thanks.

I have an idea to improve this. We should detect whether you've written your own ccallable main and in that case use it, otherwise we inject a C main that calls the julia at-main.