Fix T2C crash on Apple Silicon

[jserv]

The LLVM MCJIT backend was crashing with SIGBUS when running DOOM on ARM64 macOS. Root causes and fixes:

1. LLVM struct layout mismatch: The LLVM IR struct definition for riscv_internal had fields in wrong order (io at position 2), not matching the actual C struct layout. Fixed by reordering to match: halt, X[32], PC, timer, data, io.
2. GEP-based io function offset: t2c_gen_call_io_func used GEP with incorrect struct indices. Replaced with manual pointer arithmetic (PtrToInt -> Add -> IntToPtr) using byte offsets from offsetof(). This is robust regardless of SYSTEM mode configuration.
3. Hard-coded ecall/ebreak indices: Call sites used magic numbers that were incorrect. Changed to use offsetof(riscv_t, io) + offsetof(riscv_io_t, on_ecall/on_ebreak) for correctness across all build configurations.
4. Code model on Apple Silicon: LLVMCodeModelLarge generates movz/movk sequences that are problematic on ARM64. Use LLVMCodeModelSmall for Apple Silicon, Large for other platforms.
5. MCJIT initialization: Changed from LLVMCreateExecutionEngineForModule to LLVMCreateMCJITCompilerForModule with explicit options to ensure code model setting is respected.

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Summary by cubic

Fixes a SIGBUS crash on Apple Silicon by aligning LLVM IR with the C struct, correcting io handler address calculation, and using the Small code model on ARM64 macOS. Also switches MCJIT initialization to respect these settings.

• Bug Fixes
  • Match riscv_internal struct order in LLVM IR: halt, X[32], PC, timer, data, io.
  • Load io function pointers via byte offsets (offsetof) with PtrToInt/Add/IntToPtr; remove incorrect GEP usage.
  • Replace magic indices for ecall/ebreak with offsetof(riscv_t, io) + offsetof(riscv_io_t, on_ecall/on_ebreak).
  • Use LLVMCodeModelSmall on Apple Silicon; Large elsewhere, with PIC relocation.
  • Create the engine with LLVMCreateMCJITCompilerForModule and explicit options so the code model is applied.

^{Written for commit 7109507. Summary will update on new commits.}

[cubic-dev-ai]

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