Move some gcc extensions to new gcc_extensions.h header

- Move ATTRIBUTE_PRINTF/ATTRIBUTE_SCANF from _ansi.h
  They are not related at all to this file, and this broke compilation
  with Code Sourcery GCC which ships its own _ansi.h
- Move LIKELY/UNLIKELY from system.h

There is likely a lot more GCC extensions used everywhere in the source,
conditionally on __GNUC__ or unconditionally

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@27548 a1c6a512-1295-4272-9138-f99709370657

firmware/libc/include/stdio.h

@@ -1,14 +1,14 @@
 #ifndef _STDIO_H_
 #define _STDIO_H_
 
-#include <_ansi.h>
-
 #define __need_size_t
 #include <stddef.h>
 
 #define __need___va_list
 #include <stdarg.h>
 
+#include "gcc_extensions.h"
+
 #ifndef NULL
 #define NULL    0
 #endif