hookize function_arg_callee_copies

[Joern Rennecke]

> > Joern, please explain the rationale behind the current formation
> > of sh_callee_copies.  It looks wrong to me.
> 
> This code has actually been written by Alexandre.
> It is there for SHcompact support, and the callee copies for SHcompact are
> performed by __GCC_shcompact_incoming_args.

Hmm, we actually have caller copies here that look like and are described as
callee copies to the target independent code, but are performed by
__GCC_shcompact_call_trampoline.