Update Cabal submodule to HEAD (1.21)

[Edward Z. Yang]

The reason for this is that the builders do not have a sufficiently
recent version of process, which Cabal has upgraded to depend on.
Probably 'cabal update && cabal install process' should bring it up to
date and make it working.  Unfortunately, the build system bypasses
Cabal for the bootstrap build of Cabal, so the version dependency range
is not checked.  I suppose if you /really/ wanted to we could add a
macro to disable ctl-c support and pass it on the zeroboot.

BTW, I noticed the builders are still bootstrapping from 7.4.  Since
7.8 was stabilized recently, we're going to be retiring support for
bootstrapping from 7.4 soon. Upgrade!

Edward