<stdio.h>: Fix va_list visibility.

<stdio.h>: Fix va_list visibility.

Looks like it should be visible only at POSIX.1-2008.
SUSv2 requirement to provide va_list definition makes is harder.
While there, mention __gnuc_va_list in comment and add inclusion of <stdarg.h>
for non __GNUC__ case to prevent dports gcc "fixing" this header.