gcc-plugins: Export symbols needed by gcc

This explicitly exports symbols that gcc expects from plugins.

Based on code from Emese Revfy.

Signed-off-by: Kees Cook <keescook@chromium.org>

scripts/gcc-plugins/gcc-common.h

@@ -130,6 +130,7 @@ extern void dump_gimple_stmt(pretty_printer *, gimple, int, int);
 #endif
 
 #define __unused __attribute__((__unused__))
+#define __visible __attribute__((visibility("default")))
 
 #define DECL_NAME_POINTER(node) IDENTIFIER_POINTER(DECL_NAME(node))
 #define DECL_NAME_LENGTH(node) IDENTIFIER_LENGTH(DECL_NAME(node))