Fix issue where incoming and master SDKs were not listed in the 'emsdk list' output. Thanks to MTRo for providing the fix!

emsdk

@@ -406,10 +406,15 @@ class Tool:
   
   def compatible_with_this_os(self):
     if hasattr(self, 'os'):
+      if self.os == 'all':
+        return True
       if (WINDOWS and 'win' in self.os) or (LINUX and 'linux' in self.os) or (OSX and 'osx' in self.os):
         return True
       else:
         return False
+    else:
+      if not hasattr(self, 'osx_url') and not hasattr(self, 'windows_url') and not hasattr(self, 'unix_url'):
+        return True
 
     if OSX and hasattr(self, 'osx_url'):
       return True