feat: add support for riscv64

[ffgan]

hi there.

Recently, riscv64 has gained widespread adoption. I'm also attempting to add riscv64 support to rules_oci and other projects. One of the dependencies of rules_oci is rules_multitool here, so I hope to add riscv64 support to rules_multitool.

I performed builds and tests on my riscv64 machine, and the results show that everything went smoothly. After adding this commit, rules_oci can also support rv64 very well. Therefore, I believe we can easily add riscv64 support to rules_multitool.

So if there are any questions about the above content, please feel free to @ me directly. I'm happy to answer all questions.

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Other Info
Co-authored by: nijincheng@iscas.ac.cn;

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Restore workspace example by defining @rules_shell

In the WORKSPACE-based example, @rules_shell is never declared (checked examples/workspace/WORKSPACE.bazel), so adding load("@rules_shell//shell:sh_test.bzl", "sh_test") will make bazel test //:integration_test fail with an unresolved repository error. The previous usage relied on the native sh_test without any external repo. Either add a rules_shell repository to the workspace example or drop this load and use the native rule again.

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