[mcs] Handle try await nested in multiple finally blocks.
[corlib] Fix race in unit test
Fix Mono.Linker.csproj so it can build

Mono.Linker.csproj has drifted from its associated Makefile was was not buildable.
This change applies changes from the last few Makefile changes.
[sgen] Don't crash for describe_ptr() when there's no bridge processor.
Add a section to CONTRIBUTING.md on inactivity.