feat: make kernel architecture-agnostic; primitive work on arm port

docs/building.md

@@ -70,10 +70,16 @@ sudo apt install build-essential nasm git curl xorriso mtools gdisk qemu-system-
 
 ### Cross-Architecture Builds
 
-To build for a different architecture, pass `ARCH=`:
+The kernel is split into architecture-neutral code and per-architecture
+ports under `kernel/src/Arch/` (see [porting.md](porting.md)). x86_64 is the
+only complete port. `aarch64` is a skeleton: the whole kernel compiles and
+links for it, but paging, exception vectors, the GIC and the timer are not
+written yet, so it does not boot. Userspace is x86_64-only.
+
+Non-x86 kernels are built with LLVM (`clang`, plus `ld.lld` for the link):
 
 ```bash
-make ARCH=aarch64
-make ARCH=riscv64
-make ARCH=loongarch64
+make kernel ARCH=aarch64                    # kernel/bin-aarch64/kernel
+make kernel ARCH=aarch64 KERNEL_LD=/path/to/ld.lld
+./scripts/check-arch-neutral.sh             # boundary check + aarch64 compile
 ```