sparc64: prepare module_64.c for unification

o Introduce a helper function
o Combine sparc64 specific case values
o add ifdef's around sparc64 code snippets

Note: The ifdef around the BUG_ON is highly questionable
      but for now the safe approach was taken

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>