[AArch64] use stack symbol provided by linker for nosys

[Tamar Christina]

> > diff --git a/libgloss/aarch64/crt0.S b/libgloss/aarch64/crt0.S index
> > 3bf0278..0c669d9 100644
> > --- a/libgloss/aarch64/crt0.S
> > +++ b/libgloss/aarch64/crt0.S
> > @@ -83,7 +83,7 @@
> >  	FUNC_START	_start
> >
> >  /* Start by setting up a stack */
> > -
> > +#ifdef ARM_RDI_MONITOR
> 
> i think this should match
> 
> ...
> > +#ifdef ARM_RDP_MONITOR

We don't define ARM_RDP_MONITOR in AArch64, so this should be ARM_RDI_MONITOR.

> 
> this.
> 
> >  .LC0:
> >  	GEN_DWORD HeapBase