Need to increase 20-hour build timeout for NSS on MIPS

[Mark H Weaver <mhw@netris.org>]

NSS consistently fails to build on MIPS because it hits the 20 hour
timeout specified in 'package->alist' in build-aux/hydra/gnu-system.scm.

As a one-time hack, I modified the database on Hydra to increase the
timeout for <http://hydra.gnu.org/build/442531> to 40 hours, and found
that the build succeeded after about 28.25 hours.

The overwhelming majority of that time is spent in the test suite -- the
build phase takes only about 10 minutes on MIPS -- but I'm reluctant to
disable the test suite for a library as security-critical as NSS.

So, we either need a way to set the timeout on a per-package basis, or
we need to increase the default timeout in gnu-system.scm.  I still
don't know how much to increase it, because in this case there were no
other builds happening on the machine for much of the time, whereas
typically there will be two builds assigned to each MIPS build slave.

Suggestions?

     Mark