Add __gnuc_va_list in stdio.h to stop gcc from generating include-fixed/stdio.h

When GCC is built bionic's stdio.h (among other headers) is fixed: __va_list
is replaced with __gnuc_va_list, and put in include-fixed/stdio.h,
unless stdio.h already contains __gnuc_va_list.  When GCC is later
used bionic's stdio.h is never included.

The problem in the context of NDK is that all 32-bit Android toolchain
are built with android-9 header and expect to work with all API levels
sometime with sustantially different stdio.h.

This CL stops GCC from generating include-fixed/stdio.h by putting
__gnuc_va_list in it.

See bionic df85f50b and b.android.com/73728

Change-Id: I7a490496efbd491a0376608b9ae6e9e2dfc15fb3

ndk/platforms/android-8/include/stdio.h

@@ -38,6 +38,14 @@
 #ifndef	_STDIO_H_
 #define	_STDIO_H_
 
+/*
+ * This file must contain a reference to __gnuc_va_list so that GCC's
+ * fixincludes knows that that's what's being used for va_list, and so
+ * to leave our <stdio.h> alone. (fixincludes gets in the way of pointing
+ * one toolchain at various different sets of platform headers.)
+ * If you alter this comment, be sure to keep "__gnuc_va_list" in it!
+ */
+
 #include <sys/cdefs.h>
 #include <sys/types.h>