Add zkVM (RISC-V) cross-compilation target

[goodlyrottenapple]

Add a standalone CMake project (category/zkvm) that builds libmonad-zkvm.a for bare-metal RISC-V targets (planned ZisK and SP1 zkVM backends). This allows the Monad EVM interpreter to run inside zkVMs.

Key changes:

• RISC-V toolchain file (rv64ima, lp64, medany, -nostdlib++)
• intx-backed uint256 wrapper replacing AVX2 implementation
• zkVM-abstracted allocator (zkvm_aligned_alloc) and exit (zkvm_exit) interfaces with planned ZisK/SP1 backends
• Conditional compilation guards (#ifdef MONAD_ZKVM) for bare-metal compatibility across interpreter, runtime, and core components
• Shared compile_options.cmake extracted from root CMakeLists.txt
• Bare-metal stubs: libc (malloc/calloc/free), libstdc++ (operator new/delete, std::terminate), and assert handlers
• CI build of category/zkvm code targeting RISC-V

Co-authored with Claude

[Copilot]

Pull request overview

Adds a standalone zkVM (bare-metal RISC-V) build target to produce libmonad-zkvm.a, enabling the Monad EVM interpreter/runtime to run inside planned ZisK/SP1 zkVM backends.

Changes:

• Introduces a new category/zkvm/ CMake project, RISC-V toolchain file, and a newlib submodule for setjmp/longjmp support.
• Adds zkVM-specific libc/libstdc++ stubs plus backend-abstracted aligned_alloc and exit interfaces.
• Refactors compilation/common flags into cmake/compile_options.cmake and adds MONAD_ZKVM guards across runtime/interpreter to avoid x86/ASM dependencies.

Reviewed changes

Copilot reviewed 27 out of 27 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
third_party/newlib-cygwin	Adds newlib-cygwin submodule commit for RISC-V setjmp.S.
.gitmodules	Registers third_party/newlib-cygwin submodule.
cmake/compile_options.cmake	Extracts shared compile options; adds zkVM-specific flags/defs.
CMakeLists.txt	Reuses extracted compile_options.cmake.
category/zkvm/CMakeLists.txt	New standalone build that assembles VM object libs + zkVM stubs into libmonad-zkvm.a.
category/core/toolchains/riscv64-elf-toolchain.cmake	Adds bare-metal RISC-V toolchain configuration.
category/zkvm/include/category/zkvm/allocator.hpp	Backend-abstracted aligned allocator interface.
category/zkvm/include/category/zkvm/exit.hpp	Backend-abstracted exit syscall interface.
category/zkvm/src/libc.cpp	Minimal malloc/calloc/free stubs for bare-metal.
category/zkvm/src/libstdcxx.cpp	Minimal operator new/delete + std::terminate for bare-metal.
category/zkvm/include/category/core/runtime/uint256.hpp	intx-backed uint256_t wrapper to replace x86-specific implementation.
category/core/runtime/non_temporal_memory.hpp	Uses memcpy/memset under zkVM; AVX2 streams otherwise.
category/vm/runtime/CMakeLists.txt	Skips ASM sources/language when building for zkVM.
category/vm/runtime/types.hpp	Switches exit_stack_ptr type under zkVM to std::jmp_buf*.
category/vm/runtime/exit.cpp	Uses setjmp/longjmp-based exit under zkVM; preserves ASM exit for host.
category/vm/runtime/context.cpp	Uses zkVM allocator instead of std::aligned_alloc under zkVM.
category/vm/runtime/cached_allocator.hpp	Avoids destructor/atexit under zkVM; uses zkVM allocator.
category/vm/runtime/allocator.hpp	Removes thread_local cache under zkVM.
category/vm/runtime/allocator.cpp	Defines non-thread_local cache list under zkVM.
category/vm/runtime/transmute.hpp	Disables AVX2 bounded-load path under zkVM.
category/vm/runtime/math.hpp	Replaces ASM mul with intx multiplication under zkVM.
category/vm/interpreter/CMakeLists.txt	Skips ASM entry trampoline + utils library on zkVM.
category/vm/interpreter/execute.cpp	Bypasses trampoline and installs setjmp exit handler under zkVM.
category/vm/interpreter/instruction_table.hpp	Routes MUL to runtime::mul; avoids AVX conversion on SWAP under zkVM.
category/vm/interpreter/push.hpp	Includes <immintrin.h> only when __AVX2__ is defined.
category/vm/interpreter/debug.hpp	Compiles out tracing dependencies/output under zkVM.
category/vm/core/assert.cpp	Adds zkVM assertion handlers that exit instead of relying on libc/abort.

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