c-variadic: add a Copy bound to VaArgSafe

because a VaList can be cloned, the same c-variadic argument can be read twice and that is only safe if the argument type is copy

library/core/src/ffi/va_list.rs

@@ -333,7 +333,7 @@ impl<T> Sealed for *const T {}
 // types with an alignment larger than 8, or with a non-scalar layout. Inline assembly can be used
 // to accept unsupported types in the meantime.
 #[lang = "va_arg_safe"]
-pub unsafe trait VaArgSafe: sealed::Sealed {}
+pub unsafe trait VaArgSafe: Copy + sealed::Sealed {}
 
 crate::cfg_select! {
     any(target_arch = "avr", target_arch = "msp430") => {