Fix stack overwrite on structure return

The common code to move a returned structure packed into
registers into memory on the caller side didn't take the
register size into account when allocating local storage,
so sometimes that lead to stack overwrites (e.g. in 73_arm64.c),
on x86_64.  This fixes it by generally making gfunc_sret also return
the register size.

arm64-gen.c

@@ -1196,7 +1196,7 @@ ST_FUNC void gen_va_arg(CType *t)
     }
 }
 
-ST_FUNC int gfunc_sret(CType *vt, int variadic, CType *ret, int *align)
+ST_FUNC int gfunc_sret(CType *vt, int variadic, CType *ret, int *align, int *regsize)
 {
     return 0;
 }