This is x86 specific, and adds statefulness to
MachineModuleInfo. Instead of explicitly tracking this, infer if we
need to declare the symbol based on the reference previously inserted.
This produces a small change in the output due to the move from
AsmPrinter::doFinalization to X86's emitEndOfAsmFile. This will now be
moved relative to other end of file fields, which I'm assuming doesn't
matter (e.g. the __morestack_addr declaration is now after the
.note.GNU-split-stack part)
This also produces another small change in code if the module happened
to define/declare __morestack_addr, but I assume that's invalid and
doesn't really matter.
