Add riscv64 support

**What are you trying to do?**

With the SiFive HiFive Premier P550 there is a first RISC-V board available that supports KVM.
We should support running VMs with multipass on it.

**What's your proposed solution?**

* EDK II for riscv64 is only available in Noble and not in Jammy. To avoid booting via U-Boot upgrade to using Core24.
* Replace the virtual package qemu-efi by the real packages qemu-efi-aarch64, qemu-efi-riscv64
* Enable the riscv64 build architecture.

The development requires that the Canonical desktop team provides the Gnome SDK snap for riscv64.
Currently Gnome SDK builds for riscv64 fail in Launchpad due to a firewall issue.

**Additional context**

https://www.sifive.com/boards/hifive-premier-p550

Start by mapping Multipass's existing platform and packaging/build configuration, then check the riscv64 prerequisites described here: Core24, EDK II, qemu-efi-riscv64, and the riscv64 build architecture. Confirm whether the GNOME SDK snap dependency is available; done means VMs run on the HiFive Premier P550 with the required packages and build support.

Written by the indexing model from the issue text.