This issue is more straight-forward than #22273. ISTM, we just have to copy large values. For example, in ffi_call, we'd iterate over the arguments and alloca and memcpy the large values prior to calling ffi_call_AMD64:

    case FFI_SYSV:
      /* If a single argument takes more than 8 bytes,
         then a copy is passed by reference. */
      for (unsigned i = 0; i < cif->nargs; i++) {
          size_t z = cif->arg_types[i]->size;
          if (z > 8) {
              void *temp = alloca(z);
              memcpy(temp, avalue[i], z);
              avalue[i] = temp;
          }
      }
      /*@-usedef@*/
      return ffi_call_AMD64(ffi_prep_args, &ecif, cif->bytes,
                            cif->flags, ecif.rvalue, fn);
      /*@=usedef@*/
      break;