xplat: Improve problem handling in PAL

Claims;
- ChakraCore overall doesn't expect the system calls (i.e. threading) are failing.
- PAL does not do much to mend or provide an interface for it

Things done so far;
- #2630 (fallback) and #2679 (cache) are introduced.
- These two fixes help ChakraCore xplat tests to pass under `ulimit -n 32`

Todo;
- enable testing for limited environment to catch more issues and going forward keep it stable.
- not just `open files -n`, test for `stack size`, `vm` and `max memory`
- check, if we may have similar issues on Windows under such limitations.