[PECOFF] Assign default export ordinals in LinkingContext::verify().

Default ordinals were assigned in EdataPass, and the assigned values were
then discarded in the pass. No code other than EdataPass would not be able
to get all of the information about ordinals. That's not ideal since I'm
writing code to emit an Import Library file, which also needs ordinals.

This is a patch to move the code to assign default ordinals from EdataPass
to LinkingContext::verify(), so that assigned ordinals will be available
anywhere.

No functionality change.