Upstream Julia repo interface changes

[qinsoon]

This issue tracks our progress about upstreaming changes in Julia repo. We plan to submit two PRs to Julia: 1. introduce changes about GC interface (changes to gc.h and the new gc-common.c, etc), 2. introduce MMTk and allow people to use MMTk. Currently we focus on the first PR.

For the first PR, there are a bunch of things we need to do.

1. Clean up our fork

Essentially we would like to reduce or eliminate #ifdef MMTK_GC and #ifndef MMTK_GC outside the normal GC interface. We currently have such checks in array.c, julia_threads.c, partr.c, symbol.c, code gen, etc. We should try clean them up as much as we can.

2. Move MMTk code to the binding

We currently have a bunch of MMTk code in Julia, such as mmtk-gc.c. Those code are included in the Julia repo, as in the beginning we thought about upstreaming the binding as a part of Julia. But now it is preferable to not have the binding code in the Julia repo (MMTk will be a binary dependency for Julia). @NHDaly @d-netto Please correct me if I misunderstood anything.

3. Merge with upstream and get ready for opening a PR

The last step before we can open a PR. We do not want to merge too early, as we have many changes that easily conflicts with any upstream change.