[PATCH] Enable ARM tail calls for ELF

[Tim Northover]

> Unlikely. It'd be nice if anything that currently works with lld
> continue to work I guess? Otherwise, ld64, gnu ld, and gold should all
> continue to work. I imagine the arm bots will notice this change,
> we'll want to pay attention there.

Ok, after some talk on IRC I'm planning to commit this with one
addition tomorrow:
+ Enable tail calls by default (except on iOS < 5.0) as here.
+ Change -arm-tail-calls to -disable-arm-tail-calls until we're
confident everything works.

The effect is the same, but until we remove the command-line option
you'll be able to check whether this is the issue by passing
-disable-arm-tail-calls.

I have a soft spot for lld, so if anyone can produce a reduced test
case I'll take a look of an evening and try to fix it. Other broken
linkers are probably on their own; though loud enough complaints might
get exceptions made.

If I can convince my devices to behave long enough I'll try to do some
benchmarks too.

Tim.