[net5/illink] Add ApplyPreserveAttribute substep

[radekdoulik]

The ApplyPreserveAttribute susbtep was not (yet?) upstreamed, so
make it build and add it to the pipeline and run it before other
substeps to not break ac5269d fix.

[radekdoulik]

Let add a test for it as well.

[jonathanpeppers]

We should check is if they can actually support [Preserve] in .NET 6 for us, so we wouldn't have to bring this over.

[jonathanpeppers]

Should we change this #if to NETCOREAPP3_1 and we could remove this $(DefineConstants)?

https://github.com/xamarin/xamarin-android/blob/eadd3bda8e2a81c6ca997f05d3faac7040b29a7b/src/Microsoft.Android.Sdk.ILLink/Microsoft.Android.Sdk.ILLink.csproj#L5

If we change this .csproj to TargetFramework=net5.0 later on, then NETCOREAPP3_1 will still be defined.

[radekdoulik]

I think this was just changed recently (dotnet/sdk#13615). So NETCOREAPP3_1 will not work. I think we should use #if NET instead. I will try to use it.

[radekdoulik]

It is not defined yet in the version we are using, so I will keep NET5_LINKER around until we have the NET define.

[rolfbjarne]

For reference, this is xamarin-macios' implementation: dotnet/macios#9529

[radekdoulik]

We should check is if they can actually support [Preserve] in .NET 6 for us, so we wouldn't have to bring this over.

I would like to get this in so that we have it now. It looks like the [Preserve] might not land in upstream, at least not soon?

[jonathanpeppers]

I think we should go ahead and bring this in, and we can remove it if it makes it upstream.

[radekdoulik]

I will wait with adding test until the Full linker mode works, noted in #5132 to not forget it.