gccrs: derive: Add const generics when deriving impls

gccrs: derive: Add const generics when deriving impls

gcc/rust/ChangeLog:

	* ast/rust-ast-builder.cc (Builder::new_const_param): New function.
	* ast/rust-ast-builder.h (vec): New function for creating 3 elts vector.
	* expand/rust-derive.cc: Use the new_const_param builder.
	* ast/rust-path.h: Add get_default_value() method.