Hide various stdio implementation details.

I've left __sF exposed since that's how the OpenBSD stdin, stdout, stderr
are implemented. Other BSDs and glibc use a separate global for each instead
of an array.

Bug: 11156955
Change-Id: I9f3d2d4314a8d4a78c3197b9acd9258820c5f150

libc/stdio/glue.h

@@ -42,4 +42,8 @@ struct glue {
 	FILE	*iobs;
 };
 
+#pragma GCC visibility push(hidden)
+
 extern struct glue __sglue;
+
+#pragma GCC visibility pop