Update FEATURE_MANAGED_ETW and disable it on Unix

Update FEATURE_MANAGED_ETW and disable it on Unix

EventSource is used throughout mscorlib for tracing.  Most of EventSource's connection to Windows ETW APIs is already behind a FEATURE_MANAGED_ETW feature flag, but not all.  As a result, code that touches certain aspects of an EventSource can end up crashing as it tries to call into Windows APIs.

This commit updates places protected by the FEATURE_MANAGED_ETW flag, and then disables the flag when building mscorlib for Unix.  Eventually we're aiming to have a different implementation of EventSource on Unix that implicitly connects it to another tracing mechanism, but this change at least allows code paths that touch EventSource to function on Unix.