Add LLVM workspace

Over the next few commits, I will be splitting the LLVM project into
several subprojects, as it was getting too big for Xcode to handle.
Adding this workspace is the first step.
Move X86 targets and files into X86.xcodeproj

I also patched a few files so that they can deal with being invoked
from a changing $(SRCROOT).
Add ARM.xcodeproj