feat: support riscv64 architecture

[pl4nty]

The first dependency for running Talos on RISC-V, cc @shanduur. We've spoken about upstreaming my patches, then keeping CI/releases (and maybe bug reports?) on my forks to reduce the maintenance burden for Sidero Labs.

Not sure about terminology, maybe "experimental support for" or "enable building for"? So it's clear that official build artefacts and support won't be available.

I'm happy to remove the .kres changes too if preferred, so it's more like "support linux/riscv64 build targets".

[pl4nty]

ca-certificates and fhs appear to be circular dependencies, because they're built from pkgs using bldr. Instead of needing to bootstrap riscv64 builds in ghcr.io/siderolabs, I've just pinned them to linux/amd64 because their contents is architecture-independent. I'm happy to PR this to kres if it's suitable.

Alternatively, the images could be passed as Makefile variables and leave bootstrapping to the user.

[smira]

this file is auto-generated, so this change won't work anyways.

I don't know how riscv64 would be bootstrapped even as we don't have any hardware in any foreseeable future, nor that I'm convinced any risc is powerful enough to build Talos without cross-compiling

[pl4nty]

yeah, I left the PR as a draft because this change would need to happen in kres. it's mostly a workaround for the hardcoded ghcr.io/siderolabs.

everywhere else, I just set USERNAME and PLATFORM to pull/push with a different registry when building for riscv64. this approach worked for toolchain/tools/pkgs/talos on native hardware. it's very slow, but faster than qemu