feat: add freestanding RISC-V support, migrate to opaque pointers, and enhance test runner

[LunaStev]

1. RISC-V & Freestanding Target Support

Wave is now capable of targeting bare-metal/freestanding environments, specifically for RISC-V 64-bit systems.

• New Targets: Added FreestandingX86_64, FreestandingArm64, and FreestandingRISCV64 (riscv64-unknown-none-elf).
• RISC-V C-ABI Lowering: Implemented System V ABI rules for RV64, including complex parameter classification (ByVal/SRet) to ensure compatibility with existing C-based firmware and kernels.
• RISC-V Inline Assembly:
  • Full physical register mapping (x0-x31, a0-a7, t0-t6, etc.).
  • Support for standard constraints (r, m, i) and width calculations.
  • Automatic default clobbers for RISC-V call conventions.
• Introspection: Updated the CLI print command to list RISC-V specific CPUs and target features.

2. LLVM Opaque Pointer Migration

To align with modern LLVM standards (LLVM 15+), the backend has been migrated to use Opaque Pointers.

• Updated abi_c.rs, legacy.rs, and standard I/O generation (printf/scanf) to use context.ptr_type() instead of specific typed pointers (like i8*).
• This resolves several LLVM API deprecation warnings and prepares the compiler for future LLVM versions.

3. Advanced Test Runner (tools/run_tests.py)

The Python test suite is now much more "aware" of the host environment, preventing false failures on incompatible platforms.

• Metadata Parsing: Supports // wave-test: host-os=<os>, host-arch=<arch> headers at the top of .wave files.
• Automated Filtering: The runner now skips tests that don't match the host (e.g., Linux-specific syscall tests are skipped when running on macOS).
• Workflow Improvements:
  • Gracefully falls back to the debug build if the release binary is missing.
  • Exits with code 1 if any test fails, making it suitable for CI/CD pipelines.

4. Code Hygiene & Maintenance

• Warning Fixes: Resolved various unused variable and function warnings across the lexer, parser, and codegen.
• Dead Code: Added #[allow(dead_code)] to internal runner functions that are reserved for future debugger/REPL features.
• Safety: Removed an unnecessary unsafe block from the LLVM version fetcher.

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Benefits

• Systems Programming: RISC-V support enables Wave to be used for OS kernel and embedded firmware development.
• Modernization: Opaque pointers ensure the compiler remains compatible with the latest LLVM toolchains.
• Developer Efficiency: The smarter test runner reduces "noise" by automatically skipping irrelevant tests for the contributor's platform.