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/export/panic.h

@@ -22,7 +22,7 @@
 #ifndef __PANIC_H__
 #define __PANIC_H__
 
-#include "_ansi.h"
+#include "gcc_extensions.h"
 
 void panicf( const char *fmt, ... ) ATTRIBUTE_PRINTF(1, 2);