We define a new class of targets (MAIN_SOFTMMU_TARGETS) to cover the
major architectures. We either just build those or use the new
target-list-exclude mechanism to remove them from the list. This will
hopefully stop some of the longer builds hitting the Travis timeout
limit.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>