Add RISC-V CI build

[GiggleLiu]

Summary

• add a CI job that cross-compiles the workspace for riscv64gc-unknown-linux-gnu
• install the RISC-V GNU linker/binutils in CI
• verify the generated pred executable reports Machine: RISC-V via readelf

Verification

• cargo check --workspace --no-default-features --features ilp-lp-solvers --target riscv64gc-unknown-linux-gnu
• docker run Ubuntu/Rust simulation of the CI build and readelf verification

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.00%. Comparing base (aa2d1a1) to head (d86a1ad).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1073   +/-   ##
=======================================
  Coverage   98.00%   98.00%           
=======================================
  Files        1044     1044           
  Lines      107366   107366           
=======================================
  Hits       105228   105228           
  Misses       2138     2138           

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