Rebase of #125 (Imaging mode for native code)

[gbaraldi]

For now it's just a simple rebase + actually turning on imaging mode. There are still some issues such as const globals being turned into null pointers instead of the actual values.

[vchuravy]

Right the issue here is that the Julia C++ code itself doesn't expose the necessary information so this doesn't help with really anything. I have a PR open where I make this orthogonal from the native vs extern mode.

Essentially we need to hoist more of the loop into GPUCompiler to enable interesting use cases

[gbaraldi]

I'm not sure how necessary this is anymore. The only differences from imaging to non imagining now I think are the PTLS emission, ccalls and multiversioning.