Move SmallVector to Common

We had one implementation of this type of data structure in Arm64Emitter
and one in VideoCommon. This moves the Arm64Emitter implementation to
its own file and adds begin and end functions to it, so that VideoCommon
can use it.

You may notice that the license header for the new file is CC0. I wrote
the Arm64Emitter implementation of SmallVector, so this should be no
problem.

Source/Core/Common/CMakeLists.txt

@@ -110,6 +110,7 @@ add_library(common
   SettingsHandler.h
   SFMLHelper.cpp
   SFMLHelper.h
+  SmallVector.h
   SocketContext.cpp
   SocketContext.h
   SPSCQueue.h