Can we remove the need for the linker script to declare '__sel4_ipc_buffer_obj'?

[midnightveil]

Brought up related to #450, but we don't see any need for the linker script of the various targets to declare an IPC buffer. We believe we should be able to do exactly what we do for the stack: that is, allocate them at an arbitrary virtual address in the tool and use setvar.

microkit tool finding ipc buffer

        let ipcbuf_frame_obj_id = capdl_util_make_frame_obj(
            self,
            Fill {
                entries: [].to_vec(),
            },
            &format!("ipcbuf_{pd_name}"),
            None,
            PageSize::Small.fixed_size_bits(sel4_config) as u8,
        );
        let ipcbuf_frame_cap =
            capdl_util_make_frame_cap(ipcbuf_frame_obj_id, true, true, false, true);
        // We need to clone the IPC buf cap because in addition to mapping the frame into the VSpace, we need to bind
        // this frame to the TCB as well.
        let ipcbuf_frame_cap_for_tcb = ipcbuf_frame_cap.clone();
        let ipcbuf_vaddr = elf
            .find_symbol(SYMBOL_IPC_BUFFER)
            .unwrap_or_else(|_| panic!("Could not find {SYMBOL_IPC_BUFFER}"))
            .0;
        match map_page(
            self,
            sel4_config,
            pd_name,
            vspace_obj_id,
            ipcbuf_frame_cap,
            PageSize::Small as u64,
            ipcbuf_vaddr,
        ) {
            Ok(_) => {}
            Err(map_err_reason) => {
                return Err(format!(
                    "build_capdl_spec(): failed to map ipc buffer frame to {pd_name} because: {map_err_reason}"
                ))
            }
        };

[Ivan-Velickovic]

I would like to have the linker script do as little as possible since I generally find them confusing and easy to get wrong.

I don't know why originally this symbol was put in the linker script, at this stage I can't think of a reason why it needs to be there specifically instead of defined in libmicrokit.