[Bug #21286] enable dispatch by arity on gcc 15

Author: nobu

include/ruby/internal/anyargs.h

@@ -82,7 +82,7 @@
 #if defined(__cplusplus)
 # include "ruby/backward/cxxanyargs.hpp"
 
-#elif defined(_WIN32) || defined(__CYGWIN__)
+#elif (defined(_WIN32) || defined(__CYGWIN__)) && !RBIMPL_COMPILER_SINCE(gcc, 15, 0, 0)
 # /* Skip due to [Bug #16134] */
 
 #elif ! RBIMPL_HAS_ATTRIBUTE(transparent_union)