x264: fix build on OpenRISC

Even though x264 doesn't use an autoconf generated configure script
(and hence doesn't use the autotools-package infrastructure), it uses
config.guess and config.sub. The ones bundled in x264 are too old to
know about the OpenRISC architecture, so this commit leverages the
UPDATE_CONFIG_HOOK from the autotools-package infrastructure to update
config.sub and config.guess.

Fixes:

  http://autobuild.buildroot.net/results/b458c2dff0ecde61248f058092c6eeaa75039cfb



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>