XNNPACK tests that use mmap() fail on Hexagon devices

[steven-johnson]

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?