* gcc.dg/compat/mixed-struct-check.h, mixed-struct-defs.h,
	mixed-struct-init.h: Add 4 mixed structure types.
	struct-by-value-21_main.c, struct-by-value-21_x.c,
	struct-by-value-21_y.c, struct-return-21_main.c,
	struct-return-21_x.c, struct-return-21_y.c: New files.

From-SVN: r79538