RFR: Move Shenandoah stubs generation into ShenandoahBarrierSetAssembler

[Roman Kennke]

> *) This revert, you sure aarch64 still runs fine after it? We can take in on faith, and see what
> testing shows, but at least we should have basic idea about its safety:
> 
>   44   code_size2 = 22000           // simply increase if too small (assembler will crash if too small)

Yes, because our stubs are now compiled into a separate blob.

Will fix everything else (after coffee) and post another webrev.
Thanks, Roman