feat(codegen): `--target=<triple>` for regular (non-kernel) native builds

Problem

The backend and syscall numbers come from the compiler's own host, so a Home program cannot be cross-compiled; --target is only honored together with --kernel. packages/build/src/cross_compilation.zig exists but nothing references it.

Evidence (origin/main @ d4b219507)

• src/main.zig:3845-3866
• src/main.zig:3912
• packages/codegen/src/native_codegen.zig:3532
• packages/codegen/src/native_codegen.zig:4687
• packages/codegen/src/native_codegen.zig:5613

Scope

• Thread a target (arch + OS + ABI) through native codegen instead of builtin.target.
• Select the object format, syscall table and calling convention per target.

Acceptance criteria

• On a macOS host, home build --target=x86_64-linux produces a binary that runs in a Linux container, and vice versa.

Links

Depends on: #796 Related: #15, #583, #5 Part of #814.