isystem: ship and use stdarg.h

Ship minimal stdarg.h (1 type, 4 macros) as <linux/stdarg.h>.
stdarg.h is the only userspace header commonly used in the kernel.

GPL 2 version of <stdarg.h> can be extracted from
http://archive.debian.org/debian/pool/main/g/gcc-4.2/gcc-4.2_4.2.4.orig.tar.gz

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

include/linux/string.h

@@ -6,7 +6,7 @@
 #include <linux/types.h>	/* for size_t */
 #include <linux/stddef.h>	/* for NULL */
 #include <linux/errno.h>	/* for E2BIG */
-#include <stdarg.h>
+#include <linux/stdarg.h>
 #include <uapi/linux/string.h>
 
 extern char *strndup_user(const char __user *, long);