WIP: Emit unoptimized bitcode archive

[boomanaiden154]

This patch adds in an environment variable that can be set to a directory where unoptimized bitcode (within an archive) should be emitted during package compilation.

[boomanaiden154]

@vchuravy

This gets us what we need based on what we discussed in our meeting a couple weeks ago. However, there are a couple points that I think probably need to be fixed before it's upstream ready:

• Add this to a doc page somewhere? It's going to be a pretty esoteric option, but having information on it is probably good. I'm thinking the environment variables page should work?
• Is having this option as an environment variable okay or is there a more preferred option? I mostly chose to use an environment variable as it was easy to implement.
• Unit tests in /test/loading.jl?

Thank you so much for your assistance with all of this!

[boomanaiden154]

@vchuravy Do you mind taking a look at this when you have a chance and leeting me know what else needs to be done? Thanks!