Update Cabal submodule to tip of v1.20 branch

This corresponds to the RC of the soon-to-be Cabal 1.20 release

One noteworthy change is the removal of the `--with-ranlib` flag
requiring a small adaptation in the GHC build system.

Moreover two new licences were added, MPL and BSD2.

Due to https://github.com/haskell/cabal/issues/1622 Cabal-1.20 now
allows to strip libraries as well, this doesn't work well with
`ghc-cabal copy` being fed a `":"` strip-command argument which was
simply ignored in the past. The current code tries to retain this
semantics as backward compat. However, this needs more investigation as
I'm not sure if/why the `test_bindist` step doesn't want the libraries
to be stripped on installation.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>