Function compile_backend 

pub fn compile_backend(
    functions: &[FunctionWithCfg],
    type_pool: &TypeInternPool,
    strings: &[String],
    interner: &ThreadedRodeo,
    options: &CompileOptions,
    warnings: &[CompileWarning],
) -> MultiErrorResult<CompileOutput>

Compile analyzed functions to a binary.

This is the unified backend that handles both architectures. It:

1. Generates machine code for each function in parallel
2. Creates object files with relocations
3. Links them into an executable

This function is used by CompilationUnit::compile() and the legacy compile functions.