[nativeaot] Add more compatibility with CoreCLR and MonoVM hosts

[grendello]

Add more compatibility with the MonoVM and CoreCLR host environments to
the NativeAOT host.

It is important that the applications created for MonoVM, then potentially
migrated to CoreCLR, found themselves running in an environment as closely
resembling the one they were developed and tested in.

Add the following to the NativeAOT runtime:

• Set up environment variables found in both MonoVM and CoreCLR runtimes.
• Create XDG directories expected to exist before managed application
  code is invoked for the first time.