[bgen] Refactor a bit to not use target framework string comparison when we can use an enum. (#8018)

More future-proof, because new and different target frameworks will be
introduced in the future with .NET 5.

Also: fewer string comparisons is always good.

src/btouch.cs

@@ -323,8 +323,8 @@ public class BindingTouch {
 		if (!target_framework.HasValue)
 			throw ErrorHelper.CreateError (86);
 
-		switch (target_framework.Value.Identifier.ToLowerInvariant ()) {
-		case "xamarin.ios":
+		switch (target_framework.Value.Platform) {
+		case ApplePlatform.iOS:
 			CurrentPlatform = PlatformName.iOS;
 			nostdlib = true;
 			if (string.IsNullOrEmpty (baselibdll))
@@ -332,7 +332,7 @@ public class BindingTouch {
 			references.Add ("Facades/System.Drawing.Common");
 			ReferenceFixer.FixSDKReferences (GetSDKRoot (), "lib/mono/Xamarin.iOS", references);
 			break;
-		case "xamarin.tvos":
+		case ApplePlatform.TVOS:
 			CurrentPlatform = PlatformName.TvOS;
 			nostdlib = true;
 			if (string.IsNullOrEmpty (baselibdll))
@@ -340,7 +340,7 @@ public class BindingTouch {
 			references.Add ("Facades/System.Drawing.Common");
 			ReferenceFixer.FixSDKReferences (GetSDKRoot (), "lib/mono/Xamarin.TVOS", references);
 			break;
-		case "xamarin.watchos":
+		case ApplePlatform.WatchOS:
 			CurrentPlatform = PlatformName.WatchOS;
 			nostdlib = true;
 			if (string.IsNullOrEmpty (baselibdll))
@@ -348,7 +348,7 @@ public class BindingTouch {
 			references.Add ("Facades/System.Drawing.Common");
 			ReferenceFixer.FixSDKReferences (GetSDKRoot (), "lib/mono/Xamarin.WatchOS", references);
 			break;
-		case "xamarin.mac":
+		case ApplePlatform.MacOSX:
 			CurrentPlatform = PlatformName.MacOSX;
 			nostdlib = true;
 			if (string.IsNullOrEmpty (baselibdll)) {