Add support for RISC-V code cross compilation test

[TimePrinciple]

Add steps needed for RISC-V specific code to cross-compile.

Summary of the PR

I'm proposing using x86_64 platform to cross-compile RISC-V code and qemu-riscv64-static to test the compiled binary as a temporary solution, since RISC-V machines are yet available on BuildKite.

Requirements

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

• [x] All commits in this PR have Signed-Off-By trailers (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.
• [ ] All added/changed public-facing functionality has entries in the "Upcoming Release" section of CHANGELOG.md (if no such section exists, please create one).
• [x] Any newly added unsafe code is properly documented.

[endeneer]

@TimePrinciple Appreciate the alternative workaround. Just FYI, last time I proposed using qemu-system (see this PR) instead of qemu-user-static, because, correct me if I'm wrong, qemu-user-static has some limitations such as:

• no support for emulation of RISC-V hypervisor extension
• limited system call support (kvm_ioctls might fail)
• lack of hardware access (vfio might fail)

But for cross-compilation, qemu-user-static should suffice and looks much simpler.

[TimePrinciple]

@TimePrinciple Appreciate the alternative workaround. Just FYI, last time I proposed using qemu-system (see this PR) instead of qemu-user-static, because, correct me if I'm wrong, qemu-user-static has some limitations such as:

• no support for emulation of RISC-V hypervisor extension
• limited system call support (kvm_ioctls might fail)
• lack of hardware access (vfio might fail)

But for cross-compilation, qemu-user-static should suffice and looks much simpler.

Indeed, I'm proposing this approach because of it was mentioned in this issue.