Add riscv64 testing.
<a href="https://github.com/vnmakarov/mir/actions?query=workflow%3Aaarch64%2Dtest"><img alt="GitHub MIR test status on aarch64" src="https://github.com/vnmakarov/mir/workflows/aarch64%2Dtest/badge.svg"></a>
<a href="https://github.com/vnmakarov/mir/actions?query=workflow%3Appc64le%2Dtest"><img alt="GitHub MIR test status on ppc64le" src="https://github.com/vnmakarov/mir/workflows/ppc64le%2Dtest/badge.svg"></a>
<a href="https://github.com/vnmakarov/mir/actions?query=workflow%3As390x%2Dtest"><img alt="GitHub MIR test status on s390x" src="https://github.com/vnmakarov/mir/workflows/s390x%2Dtest/badge.svg"></a>
+<a href="https://github.com/vnmakarov/mir/actions?query=workflow%3Ariscv64%2Dtest"><img alt="GitHub MIR test status on riscv64" src="https://github.com/vnmakarov/mir/workflows/riscv64%2Dtest/badge.svg"></a>
<a href="https://github.com/vnmakarov/mir/actions?query=workflow%3AAMD64%2DLinux%2Dbench"><img alt="GitHub MIR benchmark status" src="https://github.com/vnmakarov/mir/workflows/AMD64%2DLinux%2Dbench/badge.svg"></a>
</p>
