Emit SkipLocalsInit for .NET 5

[jonpryor]

C#9 allows specifying the System.Runtime.CompilerServices.SkipLocalsInitAttribute custom attribute on methods (and other things) for performance reasons:

Finally, you can add the System.Runtime.CompilerServices.SkipLocalsInitAttribute to instruct the compiler not to emit the localsinit flag. This flag instructs the CLR to zero-initialize all local variables. The localsinit flag has been the default behavior for C# since 1.0. However, the extra zero-initialization may have measurable performance impact in some scenarios. In particular, when you use stackalloc. In those cases, you can add the SkipLocalsInitAttribute.

Our generated binding code does use stackalloc, e.g. https://github.com/xamarin/java.interop/blob/b0d170c1002e5481564c0afc78afd6dbf3d40e66/tests/generator-Tests/expected.ji/Adapters/Xamarin.Test.AdapterView.cs#L117-L129

We should consider updating generator output to place [SkipLocalsInit] on such methods, e.g.

[Register ("setAdapter", "(Lxamarin/test/Adapter;)V", "GetSetAdapter_Lxamarin_test_Adapter_Handler")]
[SkipLocalsInit]
set {
	const string __id = "setAdapter.(Lxamarin/test/Adapter;)V";
	IntPtr native_value = JNIEnv.ToLocalJniHandle (value);
	try {
		JniArgumentValue* __args = stackalloc JniArgumentValue [1];
		__args [0] = new JniArgumentValue (native_value);
		_members.InstanceMethods.InvokeAbstractVoidMethod (__id, this, __args);
	} finally {
		JNIEnv.DeleteLocalRef (native_value);
		global::System.GC.KeepAlive (value);
	}
}

Problems

The SkipLocalsInitAttribute is added in .NET 5, which places it into a similar "bucket" as SupportedOSPlatformAttribute (da12df4, a33084b), which we had to disable in 00862ad.

We likely can't emit SkipLocalsInitAttribute until we can emit SupportedOSPlatformAttribute.