0e0ee40528
Updates the symbol-operand guide: x86 now uses the same plain %[fn] as aarch64, and a 'How the portability works' note explains the mechanism (compiler auto-injects LLVM's :c modifier for "s" operands, equivalent to GCC :P/%P0 for x86 calls, no-op on aarch64, overridable). Drops the stale per-arch :P guidance; checkpoint updated.