Bring in changes from PR #8478 (#8727)

Context: #8478

Changes in this commit were originally made in #8478 but aren't
strictly tied to that PR.  To make reviews of #8478 easier, a portion
of changes were extracted and placed here.

Modify the LLVM IR generator to output more readable and compact
code:

  * Use hexadecimal integer notation wherever it makes sense (e.g.
     hashes, managed token IDs)
  * Use boolean literals `true` and `false` instead of previously used
     `1` and `0`, respectively
  * Improve formatting of arrays
  * Introduce arrays which can composed from various "sections",
     each with its own comment and with data provided during the
     generation process, instead of during the preparation phase.

Additional changes:

Extract portions of `MonoAndroidHelper.cs` and place them in a separate
file, `MonoAndroidHelper.Basic.cs` .   Code placed in there provides conversion
between RID, and various forms of Android ABI names as well as `xxHash` functions
used throughout the repository, as well as methods to handle paths of ZIP archive
entries in a uniform way.  #8478 uses these extracted methods in tests, in standalone
utilities in addition to build tasks.

Use maximum LZ4 compression level when compressing assemblies to be placed in
the application APK/AAB archives.

src/Xamarin.Android.Build.Tasks/Utilities/ApplicationConfig.cs

@@ -42,11 +42,19 @@ sealed class ApplicationConfig
 		public uint   bundled_assembly_name_width;
 		public uint   number_of_assembly_store_files;
 		public uint   number_of_dso_cache_entries;
+
+		[NativeAssembler (NumberFormat = LLVMIR.LlvmIrVariableNumberFormat.Hexadecimal)]
 		public uint   android_runtime_jnienv_class_token;
+
+		[NativeAssembler (NumberFormat = LLVMIR.LlvmIrVariableNumberFormat.Hexadecimal)]
 		public uint   jnienv_initialize_method_token;
+
+		[NativeAssembler (NumberFormat = LLVMIR.LlvmIrVariableNumberFormat.Hexadecimal)]
 		public uint   jnienv_registerjninatives_method_token;
 		public uint   jni_remapping_replacement_type_count;
 		public uint   jni_remapping_replacement_method_index_entry_count;
+
+		[NativeAssembler (NumberFormat = LLVMIR.LlvmIrVariableNumberFormat.Hexadecimal)]
 		public uint   mono_components_mask;
 		public string android_package_name = String.Empty;
 	}

src/Xamarin.Android.Build.Tasks/Utilities/ApplicationConfigNativeAssemblyGenerator.cs

@@ -29,7 +29,7 @@ public override string GetComment (object data, string fieldName)
 			{
 				var dso_entry = EnsureType<DSOCacheEntry> (data);
 				if (String.Compare ("hash", fieldName, StringComparison.Ordinal) == 0) {
-					return $" hash 0x{dso_entry.hash:x}, from name: {dso_entry.HashedName}";
+					return $" from name: {dso_entry.HashedName}";
 				}
 
 				if (String.Compare ("name", fieldName, StringComparison.Ordinal) == 0) {
@@ -49,7 +49,7 @@ sealed class DSOCacheEntry
 			[NativeAssembler (Ignore = true)]
 			public string HashedName;
 
-			[NativeAssembler (UsesDataProvider = true)]
+			[NativeAssembler (UsesDataProvider = true, NumberFormat = LlvmIrVariableNumberFormat.Hexadecimal)]
 			public ulong hash;
 			public bool ignore;
 
@@ -93,11 +93,11 @@ sealed class AssemblyStoreSingleAssemblyRuntimeData
 		// src/monodroid/jni/xamarin-app.hh AssemblyStoreRuntimeData structure
 		sealed class AssemblyStoreRuntimeData
 		{
-			[NativePointer]
+			[NativePointer (IsNull = true)]
 			public byte data_start;
 			public uint assembly_count;
 
-			[NativePointer]
+			[NativePointer (IsNull = true)]
 			public AssemblyStoreAssemblyDescriptor assemblies;
 		}
 
@@ -299,7 +299,7 @@ void HashAndSortDSOCache (LlvmIrVariable variable, LlvmIrModuleTarget target, ob
 					throw new InvalidOperationException ($"Internal error: DSO cache entry has unexpected type {instance.Obj.GetType ()}");
 				}
 
-				entry.hash = GetXxHash (entry.HashedName, is64Bit);
+				entry.hash = MonoAndroidHelper.GetXxHash (entry.HashedName, is64Bit);
 			}
 
 			cache.Sort ((StructureInstance<DSOCacheEntry> a, StructureInstance<DSOCacheEntry> b) => a.Instance.hash.CompareTo (b.Instance.hash));

src/Xamarin.Android.Build.Tasks/Utilities/AssemblyCompression.cs

@@ -78,7 +78,7 @@ public static CompressionResult Compress (AssemblyData data, string outputDirect
 				}
 
 				destBytes = bytePool.Rent (LZ4Codec.MaximumOutputSize (sourceBytes.Length));
-				int encodedLength = LZ4Codec.Encode (sourceBytes, 0, checked((int)fi.Length), destBytes, 0, destBytes.Length, LZ4Level.L09_HC);
+				int encodedLength = LZ4Codec.Encode (sourceBytes, 0, checked((int)fi.Length), destBytes, 0, destBytes.Length, LZ4Level.L12_MAX);
 				if (encodedLength < 0)
 					return CompressionResult.EncodingFailed;
 

src/Xamarin.Android.Build.Tasks/Utilities/LlvmIrGenerator/LlvmIrComposer.cs

@@ -1,7 +1,5 @@
 using System;
 using System.IO;
-using System.IO.Hashing;
-using System.Text;
 
 using Microsoft.Build.Utilities;
 
@@ -41,18 +39,13 @@ public void Generate (LlvmIrModule module, AndroidTargetArch arch, StreamWriter
 			LlvmIrGenerator generator = LlvmIrGenerator.Create (arch, fileName);
 			generator.Generate (output, module);
 			output.Flush ();
-		}
 
-		public static ulong GetXxHash (string str, bool is64Bit)
-		{
-			byte[] stringBytes = Encoding.UTF8.GetBytes (str);
-			if (is64Bit) {
-				return XxHash64.HashToUInt64 (stringBytes);
-			}
-
-			return (ulong)XxHash32.HashToUInt32 (stringBytes);
+			CleanupAfterGeneration (arch);
 		}
 
+		protected virtual void CleanupAfterGeneration (AndroidTargetArch arch)
+		{}
+
 		protected LlvmIrGlobalVariable EnsureGlobalVariable (LlvmIrVariable variable)
 		{
 			var gv = variable as LlvmIrGlobalVariable;