Remove LLVM GC patches, add ARM patches, remove clang, rebase onto trunk

[brson]

We have several LLVM changes in the pipeline. Let's do them all at once to avoid too much churn.
#5327 has a number of changes to support __morestack on ARM. Today I'm going to take that branch and try to rebase out our GC patches.

@yichoi (or @ILyoan - I can't recall) also mentioned on Wednesday that they want to update LLVM to trunk to pick up some ARM fixes.

@ILyoan @yichoi are there any other modifications you expect to LLVM before 0.6?

[brson]

Probably should also incorporate #3775 once we rebase to trunk.

[brson]

And #5403

[brson]

Dupe of #5368