How to use D for cross platform development?

[Bennie Copeland]

On Sunday, 25 March 2012 at 16:20:51 UTC, Alex Rønne Petersen 
wrote:
>
> About ARM support: That's not strictly true. D on ARM should 
> work fine at this point in time if you build druntime/phobos in 
> GDC with -fno-section-anchors (there is even some experimental 
> Android support). But indeed, no iOS support.
>

I'm still new at low level programming topics like ABI's etc, so 
I'll betray my ignorance. Is the druntime only required for 
providing an interface between the executable and OS resources 
like IO, or is it inseparable from the language? For example, if 
I wrote the shell of an iOS app in Obj-C that handled the 
interface with the phone software/hardware, could the remaining 
core be written using D? Or is the language capabilities like GC, 
slicing, mixins, etc. intimately tied to the runtime or phobes?