[CoreCLR] Remove unused marshal methods code (#10180)

Our build process targetting CoreCLR generated marshal methods code that is
only used by our MonoVM runtime. The native code included marshal methods and 
class names, as well as assembly image cache which is not supported on CoreCLR.

All of the above data were stored as pointers in the resulting `libxamarin-app.so`,
each pointer being a relocation that has to be fixed up by the system dynamic linker
at application load time. The more relocations exist across the `.so` file, the more
of it the linker has to actually read into memory (instead of just mmapping it and 
lazily loading whenever/if the code/data in that section is used).

This PR removes all of that code from the CoreCLR version of `libxamarin-app.so`,
together with a number of native structs supporting the data.

Testing the changes on a `dotnet new maui -sc` application, we can see the following
savings:

  * Number of relocations in `libxamarin-app.so` drops from 903 to 394 entries
  * `libxamarin-app.so` size drops from:
    * arm64: 443648 to 368832 bytes (16.9%)
    * x64: 438856 to 360464 bytes (17.9%)

Testing the above MAUI app performance on Pixel 8, we can also see a tiny startup
time decrease by 0.71%. On slower devices and with bigger applications the startup
performance improvement will be bigger (the slower the device's storage, the bigger
time savings)

Author: grendello

src/Xamarin.Android.Build.Tasks/Tasks/GenerateNativeMarshalMethodSources.cs

@@ -29,14 +29,20 @@ public class GenerateNativeMarshalMethodSources : AndroidTask
 	[Required]
 	public ITaskItem [] ResolvedAssemblies { get; set; } = [];
 
+	[Required]
+        public string AndroidRuntime { get; set; } = "";
+
 	public ITaskItem [] SatelliteAssemblies { get; set; } = [];
 
 	[Required]
 	public string [] SupportedAbis { get; set; } = [];
 
+	AndroidRuntime androidRuntime;
+
 	public override bool RunTask ()
 	{
 		NativeCodeGenStateCollection? nativeCodeGenStates = null;
+		androidRuntime = MonoAndroidHelper.ParseAndroidRuntime (AndroidRuntime);
 
 		if (EnableMarshalMethods) {
 			// Retrieve the stored NativeCodeGenStateCollection (and remove it from the cache)
@@ -60,24 +66,11 @@ void Generate (NativeCodeGenStateCollection? nativeCodeGenStates, string abi)
 		var marshalMethodsLlFilePath = $"{marshalMethodsBaseAsmFilePath}.ll";
 		var (assemblyCount, uniqueAssemblyNames) = GetAssemblyCountAndUniqueNames ();
 
-		MarshalMethodsNativeAssemblyGenerator marshalMethodsAsmGen;
-
-		if (EnableMarshalMethods) {
-			marshalMethodsAsmGen = new MarshalMethodsNativeAssemblyGenerator (
-				Log,
-				assemblyCount,
-				uniqueAssemblyNames,
-				EnsureCodeGenState (nativeCodeGenStates, targetArch),
-				EnableManagedMarshalMethodsLookup
-			);
-		} else {
-			marshalMethodsAsmGen = new MarshalMethodsNativeAssemblyGenerator (
-				Log,
-				targetArch,
-				assemblyCount,
-				uniqueAssemblyNames
-			);
-		}
+		MarshalMethodsNativeAssemblyGenerator marshalMethodsAsmGen = androidRuntime switch {
+			Tasks.AndroidRuntime.MonoVM => MakeMonoGenerator (),
+			Tasks.AndroidRuntime.CoreCLR => MakeCoreCLRGenerator (),
+			_ => throw new NotSupportedException ($"Internal error: unsupported runtime type '{androidRuntime}'")
+		};
 
 		var marshalMethodsModule = marshalMethodsAsmGen.Construct ();
 		using var marshalMethodsWriter = MemoryStreamPool.Shared.CreateStreamWriter ();
@@ -88,6 +81,44 @@ void Generate (NativeCodeGenStateCollection? nativeCodeGenStates, string abi)
 			marshalMethodsWriter.Flush ();
 			Files.CopyIfStreamChanged (marshalMethodsWriter.BaseStream, marshalMethodsLlFilePath);
 		}
+
+		MarshalMethodsNativeAssemblyGenerator MakeMonoGenerator ()
+		{
+			if (EnableMarshalMethods) {
+				return new MarshalMethodsNativeAssemblyGeneratorMonoVM (
+					Log,
+					assemblyCount,
+					uniqueAssemblyNames,
+					EnsureCodeGenState (nativeCodeGenStates, targetArch),
+					EnableManagedMarshalMethodsLookup
+				);
+			}
+
+			return new MarshalMethodsNativeAssemblyGeneratorMonoVM (
+				Log,
+				targetArch,
+				assemblyCount,
+				uniqueAssemblyNames
+			);
+		}
+
+		MarshalMethodsNativeAssemblyGenerator MakeCoreCLRGenerator ()
+		{
+			if (EnableMarshalMethods) {
+				return new MarshalMethodsNativeAssemblyGeneratorCoreCLR (
+					Log,
+					uniqueAssemblyNames,
+					EnsureCodeGenState (nativeCodeGenStates, targetArch),
+					EnableManagedMarshalMethodsLookup
+				);
+			}
+
+			return new MarshalMethodsNativeAssemblyGeneratorCoreCLR (
+				Log,
+				targetArch,
+				uniqueAssemblyNames
+			);
+		}
 	}
 
 	(int assemblyCount, HashSet<string> uniqueAssemblyNames) GetAssemblyCountAndUniqueNames ()