Add CI

[ojeda]

Adds CI:

• Includes runtime validation via QEMU with a few simple tests on the system log from outside the VM.
• Test matrix (debug, release) x (builtin, loadable).
  • Release is a very minimal kernel config to speed the CI up in both build and QEMU phases (currently full CI takes just 9 minutes).
  • Debug is the same but enables pretty much all debug options that we likely care about -- the idea here is to try to catch as many mistakes as possible, e.g. on bad usage of kernel APIs from the Rust side, on corruption due to bad codegen/bindings/ABI/... and unsafe code, etc.

Later on we may want to enable another dimension for GCC vs. Clang, LLVM=1, etc. if/whenever they start working.

The PR also contains a small fix: kernel now only exports sysctl.rs only if CONFIG_SYSCTL is enabled (found out thanks to the very minimal config used) -- otherwise we fail compilation because bindgen does not generate the required bits.

[ojeda]

Thanks Alex, as usual!