Initial support for riscv64 Buildkite CI

[endeneer]

Summary of the PR

• Enable build-gnu-riscv64 and unittests-gnu-riscv64
• To be used with the container in https://github.com/rust-vmm/rust-vmm-container/pull/91
• Need help from Docker Hub admin to create a new repository rustvmm/dev_riscv64 (see https://github.com/rust-vmm/rust-vmm-container/issues/96). The checks below fail due to not having such repository.
• Need help from Buildkite admin to update the pipeline

The riscv64-related outputs of the autogenerate_pipeline.py:

- label: build-gnu-riscv64
  command: RUSTFLAGS="-D warnings" cargo build --release --target=riscv64gc-unknown-linux-gnu
    --config target.riscv64gc-unknown-linux-gnu.linker=\"riscv64-linux-gnu-gcc\"
  retry:
    automatic: false
  agents:
    os: linux
    platform: x86_64.metal
    hypervisor: kvm
  plugins:
  - docker#v5.3.0:
      image: rustvmm/dev_riscv64:v1
      always-pull: true
  timeout_in_minutes: 15

and

- label: unittests-gnu-riscv64
  command: cargo test --no-run --target=riscv64gc-unknown-linux-gnu --config target.riscv64gc-unknown-linux-gnu.linker=\"riscv64-linux-gnu-gcc\"
    && qemu.sh
  retry:
    automatic: false
  agents:
    os: linux
    platform: x86_64.metal
    hypervisor: kvm
  plugins:
  - docker#v5.3.0:
      image: rustvmm/dev_riscv64:v1
      always-pull: true
  timeout_in_minutes: 15

Requirements

Before submitting your PR, please make sure you addressed the following requirements:

• [x] All commits in this PR are signed (with git commit -s), and the commit message has max 60 characters for the summary and max 75 characters for each description line.
• [x] All added/changed functionality has a corresponding unit/integration test.
• [N/A] All added/changed public-facing functionality has entries in the "Upcoming Release" section of CHANGELOG.md (if no such section exists, please create one).
• [N/A] Any newly added unsafe code is properly documented.