tools/nolibc: annotate va_list printf formats

__attribute__(format(printf)) can also be used for functions that take a
va_list argument.

As per the GCC docs:

    For functions where the arguments are not available to be checked
    (such as vprintf), specify the third parameter as zero.

Link: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>