Remove std from libziskos.a

[marcinbugaj]

Motivation

libziskos.a is a C static archive. As a staticlib crate, the Rust toolchain bundles all transitive object files into it — including those from any dependency that pulls in std. Before this change, zkvm-interface caused 16+ std CGU object files to be included, because its bindgen-generated bindings referenced ::std::os::raw::c_int.

Changes

zkvm-interface: Adding .use_core() to the bindgen builder switches the generated C types from ::std::os::raw to ::core::ffi, after which the crate compiles without std and #![no_std] can be added.

zisk-definitions: Contains only const declarations with no std usage; #![no_std] is added trivially.

ziskos — panic handler: Making zkvm-interface no_std removed the last dependency that implicitly linked the Zisk custom std, which had been providing the #[panic_handler] lang item that the compiler requires for every no_std static library. A panic handler is added to ziskos inside the existing zkvm-gated module. It writes the panic message to UART then calls core::intrinsics::abort(), which the Zisk LLVM backend lowers to the RISC-V unimp instruction — matching the abort_internal() implementation in the Zisk standard library. This handler serves both consumers of ziskos: Rust crate dependents (via the rlib) and C programs linking the archive (where rust_begin_unwind must be resolvable from within the archive itself).

CI check

A step is added to the test-x86_64 job that builds ziskos for the zkvm target and inspects libziskos.a with ar t. Since Rust embeds the crate name in codegen unit filenames, any bundled std object file is immediately identifiable by name. The check fails the build and prints the offending filenames if any are found.