list command: can now filter results by supported platform

Added -p option to the list command. This alows filtering results by
supported platforms, e.g.

	wa list workloads -p linux

Also adding missing supported_platforms attribute to various extensions.
If an extension does not have this attribute, the assumption is that it
is supported by all available platforms.

wlauto/workloads/dex2oat/__init__.py

@@ -39,6 +39,7 @@ class Dex2oatBenchmark(Workload):
 
         """
 
+    supported_platforms = ['android']
     command_template = 'dex2oat --dex-file={} --oat-file={} --instruction-set={} --dump-timing'
     run_timeout = 5 * 60