[NDK] Properly detect 64-bit NDK

The code detecting whether an instance of NDK is a 64-bit one assumed
that the GNU binutils paths existed below the toolchains dir and used
them to check whether the 64-bit platform directory exists under them in
order to determine if the NDK is a 64-bit one. NDK r23, however, will
remove the GNU binutils completely and those paths no longer exist.
Add a check for the `toolchains/llvm/prebuilt/[PLATFORM]` directory
existence before the older check so that detection works correctly for
NDK r23+.

Author: grendello

src/Xamarin.Android.Tools.AndroidSdk/Sdks/AndroidSdkBase.cs

@@ -87,7 +87,11 @@ public virtual void Initialize (string? androidSdkPath = null, string? androidNd
 if(!string.IsNullOrEmpty(AndroidNdkPath)){
 // It would be nice if .NET had real globbing support in System.IO...
 stringtoolchainsDir=Path.Combine(AndroidNdkPath,"toolchains");
-if(Directory.Exists(toolchainsDir)){
+stringllvmNdkToolchainDir=Path.Combine(toolchainsDir,"llvm","prebuilt",NdkHostPlatform64Bit);
+
+if(Directory.Exists(llvmNdkToolchainDir)){
+IsNdk64Bit=true;
+}elseif(Directory.Exists(toolchainsDir)){
 IsNdk64Bit=Directory.EnumerateDirectories(toolchainsDir,"arm-linux-androideabi-*")
 .Any(dir =>Directory.Exists(Path.Combine(dir,"prebuilt",NdkHostPlatform64Bit)));
 }