Also compute Haskell tags in "make tags"

Note however that this won't work correctly with older compilers, due
to ghc issue #4256; however, it's a handy way to build complete a TAGS
file for Emacs.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>