D152712 replaced llvm::sys::RetryAfterSignal(-1, ::open) with
FileSystem::Instance().Open() for bionic and FileSystem::Instance() is
failing with the re-initialization assertion on arm Linux test job.
This is because MainLoopTest does not tear down the FileSystem instance.
https://lab.llvm.org/buildbot/#/builders/96/builds/40781
Add FileSystem to SubsystemRAII<> to ensure destroying the FileSystem instance.