Add RISC-V target with zero runtime capability

[konard]

Summary

This PR implements RISC-V target support with zero runtime capability as requested in issue #2.

Features Implemented

✅ RISC-V 32-bit target triple (riscv32i-unknown-none-elf)

• Supports compilation to bin, dylib, and staticlib output types
• Uses bare metal environment (os = "none")
• Configured for RISC-V 32-bit architecture with appropriate data layout
• Suitable for embedded systems and zero-runtime environments

✅ Optional entry point via -Z no_main flag

• When enabled, entry_fn returns None
• Allows compilation without main or #[start] functions
• Enables building libraries and runtime-less code

✅ #[no_mangle] attribute support

• Functions marked with #[no_mangle] preserve their original symbol names
• Symbol mangling is skipped for such functions
• Essential for FFI and bare metal programming

Implementation Details

New Files

• compiler/middle/src/spec/base/none.rs - Bare metal base configuration
• compiler/middle/src/spec/targets/riscv32i_unknown_none_elf.rs - RISC-V target specification
• experiments/test_riscv_*.zxc - Test files demonstrating functionality

Modified Files

• compiler/middle/src/symbol.rs - Added no_mangle symbol
• compiler/middle/src/man/mod.rs - Skip mangling for #[no_mangle] functions
• compiler/middle/src/hir/mod.rs - Respect no_main flag in entry_fn()
• compiler/middle/src/sess/mod.rs - Added -Z no_main compiler option
• compiler/middle/src/spec/mod.rs - Registered new RISC-V target

Additional Changes

• Updated project to Rust 2024 edition for compatibility with let chains
• Removed stabilized feature flags

Usage Examples

Compile for RISC-V with no main function

zxc --target riscv32i-unknown-none-elf -Z no_main --crate-type staticlib input.zxc

Use #[no_mangle] attribute

#[no_mangle]
fn my_function() -> i32 {
    42
}

Testing

Test files have been added to the experiments/ directory demonstrating:

1. Zero-runtime code with #[no_mangle] functions
2. Mixed code with both #[no_mangle] functions and a main entry point

Related

Fixes #2

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[konard]

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