By providing Occlum PAL as a shared library, it is now possible to embed and
use Occlum in an user-controled process (instead of an Occlum-controlled one).
The APIs of Occlum PAL can be found in `src/pal/include/occlum_pal_api.h`. The
Occlum PAL library, namely `libocclum-pal.so`, can be found in `.occlum/build/lib`.
To use the library, check out the source code of `occlum-run` (under
`src/run`), which can be seen as a sample code for using the Occlum PAL
library.
