gitlab-ci: Split the world build into 4 jobs

To better allow for parallelization of the world build job split things
into 32bit ARM (687 boards), 64bit ARM (215), PowerPC (311 boards) and
everything else (167 boards).

While the 32bit ARM job is heavier than I would like, there is not a
natural split that would reduce it in half or so without requiring the
sort of hard to maintain splits we have to do in Travis CI.

Signed-off-by: Tom Rini <trini@konsulko.com>