C#: Let platforms signal if they support it or not

Instead of hardcoding platform names that support C#, let platforms
set a flag indicating if they support it. All public platforms
except web already support it, and it's a pain to maintain a patch
for this list just to add additional names of proprietary console
platforms.

This makes adding new platforms or variants or existing platforms
much easier, as the platform can signal what it supports/doesn't
support directly, and we can avoid harcoding platform names.

platform/linuxbsd/detect.py

@@ -65,6 +65,7 @@ def get_doc_path():
 def get_flags():
     return [
         ("arch", detect_arch()),
+        ("supported", ["mono"]),
     ]