[dotnet] Change the order of the linker steps. Fixes #17347. (#17360)

The bug manifests like this:

> Could not create an native instance of the type WindowsAzure.Messaging.SBNotificationHub: the native class hasn't been loaded.

which happens because the SBNotificationHub doesn't exist in the final
executable. We asked the linker to link with the static library containing
this type, but the linker didn't link with the library because it didn't need
any of the symbols in it.

We should have collected all the exported Objective-C types from this library
and asked the native linker to keep them, but that didn't happen because:

1. We collect bound Objective-C classes from binding libraries here (the
   ListExportedSymbolsStep): https://github.com/xamarin/xamarin-macios/blob/608765e2c9e474c494958844a94354f90fa42f36/tools/linker/MonoTouch.Tuner/ListExportedSymbols.cs#L148-L157

2. That only happens for attributes with a LinkWith attribute.
	* We compute if an assembly has a LinkWith attribute here:
	  https://github.com/xamarin/xamarin-macios/blob/608765e2c9e474c494958844a94354f90fa42f36/tools/common/Assembly.cs#L266
	* Which is called from here:
	  https://github.com/xamarin/xamarin-macios/blob/608765e2c9e474c494958844a94354f90fa42f36/tools/common/Assembly.cs#L198
	* Which is called from here (the ExtractBindingLibrariesStep):
	  https://github.com/xamarin/xamarin-macios/blob/608765e2c9e474c494958844a94354f90fa42f36/tools/dotnet-linker/Steps/ExtractBindingLibrariesStep.cs#L18

Now, we must obviously compute if an assembly has a LinkWith attribute before
doing anything that depends on that value, but we weren't doing things in that
order.

Changing the custom linker steps to run the ListExportedSymbols step *after*
the ExtractBindingLibrariesStep fixes this logic problem.

Fixes #17347.

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -605,9 +605,10 @@
 			<!--
 				pre-output custom steps
 			-->
-			<_TrimmerCustomSteps Include="$(_AdditionalTaskAssembly)" BeforeStep="OutputStep" Type="Xamarin.Linker.Steps.ListExportedSymbols" />
 			<_TrimmerCustomSteps Include="$(_AdditionalTaskAssembly)" BeforeStep="OutputStep" Type="Xamarin.Linker.LoadNonSkippedAssembliesStep" />
 			<_TrimmerCustomSteps Include="$(_AdditionalTaskAssembly)" BeforeStep="OutputStep" Type="Xamarin.Linker.ExtractBindingLibrariesStep" />
+			<!-- The ListExportedSymbols must run after ExtractBindingLibrariesStep, otherwise we won't properly list exported Objective-C classes from binding libraries -->
+			<_TrimmerCustomSteps Include="$(_AdditionalTaskAssembly)" BeforeStep="OutputStep" Type="Xamarin.Linker.Steps.ListExportedSymbols" />
 			<_TrimmerCustomSteps Include="$(_AdditionalTaskAssembly)" BeforeStep="OutputStep" Type="Xamarin.Linker.Steps.PreOutputDispatcher" />
 
 			<!--