CI: Add a CI to build ELF

[Arielfoever]

just for some... You can use it in https://github.com/riscv/sail-riscv/tree/master/test/riscv-tests

[trdthg]

@aswaterman Any suggestions?

Our starting point is that we want to integrate riscv-tests tests into sail-riscv. Our options are as follows:

• download test binary from ci release
  • This is what this PR does. Downloading test files in CI maybe is not a good idea. Although we could cache it in CI.
• using riscv-tests as submodule, and build it in ci
  • I personally think this method is the best, although I don't think git submodule is particularly easy to use.
• build or download binary and keep it in reposity.
  • Easiest method for users, they don't need to configure riscv toolchain or anything else locally, but the disadvantage is also obvious, it takes up a lot of disk space

But no matter which option, I think there is no harm in distributing binaries in this repository.