[PATCH] Enable ARM tail calls for ELF

[Tim Northover]

>>> // This option should go away when tail calls fully work.

> That was almost assuredly Evan, might want to ask him what problems he
> thinks there still might be? :)

Bob, actually. It looks like there was a problem with some linker back
in the day (my guess is the MCJIT linker since it tends to be more
ad-hoc than static linkers, but I haven't checked).

These days, I think the LLVM-only parts have probably been adequately
tested by iOS, which leaves random home-grown linkers that may not
cope. I think that's their problem but wanted to make sure no-one had
one that they thought was important enough to block LLVM with.

I suppose there could be a disagreement about the functions where it's
acceptable to change the ABI for a tail call between ELF and MachO,
but that seems unlikely to me.

Tim.