XNNPACK tests that use mmap() fail on Hexagon devices

Some of our tests (eg. for testing code cache) use mmap()/munmap()/etc for memory management. On the simulator, these tests appear to run just fine, but on a Samsung S22, these tests fail at load time; logcat output shows messages of the form `symbol mmap is blacklisted`. Are these functions unsupported on QuRT on this hardware? Or is there some additional privilege (signing etc) needed to use them?