luajit: fix build II

* previous fix didn't work very well, because host version really needs
  to use the same pointer size, so it's better to disable native builds
  completely and let HOST_LUA to be compiled in target build with
  correct params
* EXTRA_OEMAKE_append_mips64 = ' HOST_CC="${BUILD_CC} -m32"'
  looks like mistake, is mips64 using 32bit pointers?

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>