[variadic] preserve former assigns order to reduce oracle differences

src/plugins/variadic/standard.ml

src/plugins/variadic/tests/known/oracle/print_libc.pretty.c

src/plugins/variadic/tests/known/oracle/printf.res.oracle

...variadic/tests/known/oracle/printf_garbled_mix.res.oracle

...variadic/tests/known/oracle/printf_wrong_arity.res.oracle

...iadic/tests/known/oracle/printf_wrong_pointers.res.oracle

...variadic/tests/known/oracle/printf_wrong_types.res.oracle

src/plugins/variadic/tests/known/oracle/scanf.res.oracle

...plugins/variadic/tests/known/oracle/scanf_loop.res.oracle

...lugins/variadic/tests/known/oracle/scanf_wrong.res.oracle

src/plugins/variadic/tests/known/oracle/snprintf.res.oracle

...lugins/variadic/tests/known/oracle/stdio_print.res.oracle

...plugins/variadic/tests/known/oracle/stdio_scan.res.oracle

src/plugins/variadic/tests/known/oracle/swprintf.res.oracle

src/plugins/variadic/tests/known/oracle/wchar.res.oracle