s390: Reorganize assembly

Avoid using ffi_prep_args as a callback; do all the work setting
up the frame within ffi_call_int directly.  Save fewer registers
in ffi_closure_SYSV.

src/s390/internal.h

@@ -0,0 +1,11 @@
+/* If these values change, sysv.S must be adapted!  */
+#define FFI390_RET_DOUBLE	0
+#define FFI390_RET_FLOAT	1
+#define FFI390_RET_INT64	2
+#define FFI390_RET_INT32	3
+#define FFI390_RET_VOID		4
+
+#define FFI360_RET_MASK		7
+#define FFI390_RET_IN_MEM	8
+
+#define FFI390_RET_STRUCT	(FFI390_RET_VOID | FFI390_RET_IN_MEM)