iOS BCL: Build dummy Xamarin.iOS.dll which is necessary

Fixes godotengine/godot#42960

bcl.py

@@ -178,6 +178,25 @@ def make_product(opts: BclOpts, product: str):
 
         run_command('csc', android_env_csc_args)
 
+    # (custom 'Xamarin.iOS.dll')
+    if product == 'ios':
+        this_script_dir = os.path.dirname(os.path.realpath(__file__))
+        monotouch_profile_dir = '%s/%s' % (install_dir, 'monotouch')
+        refs = ['mscorlib.dll', 'System.Net.Http.dll']
+
+        mkdir_p(monotouch_profile_dir)
+
+        android_env_csc_args = [
+            path_join(this_script_dir, 'files', 'xi.cs'),
+            '-keyfile:' + path_join(this_script_dir, 'files', 'xi.snk'),
+            '-out:%s' % path_join(monotouch_profile_dir, 'Xamarin.iOS.dll'),
+            '-optimize', '-deterministic', '-publicsign', '-target:library',
+            '-nostdlib', '-noconfig', '-langversion:latest'
+        ]
+        android_env_csc_args += ['-r:%s' % path_join(monotouch_profile_dir, r) for r in refs]
+
+        run_command('csc', android_env_csc_args)
+
 
 def clean_product(opts: BclOpts, product: str):
     clean_bcl(opts)

files/xi.cs

@@ -0,0 +1,15 @@
+using System.Net.Http;
+
+[assembly:System.Reflection.AssemblyVersionAttribute ("0.0.0.0")]
+[assembly:System.Runtime.CompilerServices.InternalsVisibleTo ("System.Net.Http, PublicKey=002400000480000094000000060200000024000052534131000400000100010007d1fa57c4aed9f0a32e84aa0faefd0de9e8fd6aec8f87fb03766c834c99921eb23be79ad9d5dcc1dd9ad236132102900b723cf980957fc4e177108fc607774f29e8320e92ea05ece4e821c0a5efe8f1645c4c0c93c1ab99285d622caa652c1dfad63d745d6f2de5f17e5eaf0fc4963d261c8a12436518206dc093344d5ad293")]
+
+namespace ObjCRuntime
+{
+    internal class RuntimeOptions
+    {
+        internal static HttpMessageHandler GetHttpMessageHandler()
+        {
+            return new HttpClientHandler();
+        }
+    }
+}