supports_c_variadic_definitions: extend checklist for new targets

Author: RalfJung

compiler/rustc_target/src/spec/mod.rs

@@ -2226,6 +2226,8 @@ impl Target {
             // `clang`, or we should understand and document why it deviates.
             // - Ensure that `va_arg` is implemented in rustc. For stable targets we don't rely on
             // the LLVM implementation, it has historically caused miscompilations.
+            // - Ensure that LLVM's `va_end` for this target is a NOP.
+            // - Ensure that LLVM's `va_copy` for this target is equivalent to `memcpy`.
             // - The `tests/ui/c-variadic/roundtrip.rs` test must pass for the target. It may
             // need slight modifications for embedded targets, that's fine.
             // - Check that calling c-variadic functions defined in Rust can be called from C.