Update Cabal submodule to HEAD (1.21)

[Edward Z. Yang]

For the record, Cabal is now up-to-date, we're using a different
patch which relaxes the version constraint on process so that 7.6
bootstraps.

Cheers,
Edward

Excerpts from Joachim Breitner's message of 2014-07-21 08:22:32 +0100:
> Hi,
> 
> Am Sonntag, den 20.07.2014, 22:57 +0100 schrieb Edward Z.Yang:
> > Since this patch causes GHC HEAD to not bootstrap out of the box
> > from GHC 7.6, I've reverted it for now.  We'll have to cross
> > this bridge sometime though.
> 
> thanks.
> 
> Cabal already has applied the patch to make the initial ghc-cabal binary
> build:
> https://github.com/haskell/cabal/commit/3ef560208721a050e91fd9e67a0066ce44b04ba2
> 
> Now all we need to do is to figure out how to tell the build system to
> build process before Cabal. I started in
> http://www.haskell.org/pipermail/ghc-devs/2014-July/005685.html
> but that didn’t satisfiy the build system completely. Probably simply,
> but I have been staring at the makefiles for too long already.
> 
> Greetings,
> Joachim
>