Update ghc-cabal command line usage text.
Update ghc-cabal command line usage text.

Summary:
Old usage text was horribly out-of-date.  Now updated!

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: none

Reviewers: bgamari, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2889

ezyang/ghc,olsner/ghc,olsner/ghc,sdiehl/ghc,ezyang/ghc,olsner/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,olsner/ghc,sdiehl/ghc,sdiehl/ghc,olsner/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,olsner/ghc,olsner/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc