Modify result allocation in cfunc_call.
Modify result allocation in cfunc_call.

According to libffi man page, 3rd parameter of ffi_call must poiont to storage that is sizeof(long) or larger.

NOTICE:
 This is temporary patch because this patch will not work on Big-endian machine.

mobiruby/mruby-cfunc,mobiruby/mruby-cfunc