Improve CI coverage

[bjorn3]

• debug mode build (Linux only)
• lld (Linux only)
• CG_CLIF_FUNCTION_SECTIONS=1 -Zfunction-sections=yes (Linux and macOS)
• bootstrap rustc (Test bootstrapping of rustc using cg_clif #1105)
• run rustc test suite (Run the rustc test suite on CI #1149)
• without unstable cg_clif features (bjorn3@f296311)