xplat: rand_s fallback
xplat: rand_s fallback

- Fixes a failing node-chakracore test.
   In case of an IO starvation fallback to a weaker rand instead of fatal-crash.

- Discards RND cache in case of previously broken IO.

Microsoft/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore