stdio: make internal symbols static/hidden

Although header libc/stdio/local.h declares the macros and private
variables of stdio, there are several internal symbols exposed
unexpectedly.

Change-Id: Ie7a07f85b70322fb9cd05b3c8e1bcc416061eb4b

libc/stdio/fvwrite.c

@@ -44,7 +44,7 @@
  * This routine is large and unsightly, but most of the ugliness due
  * to the three different kinds of output buffering is handled here.
  */
-int
+__LIBC_HIDDEN__ int
 __sfvwrite(FILE *fp, struct __suio *uio)
 {
 	size_t len;