At the same time I've grouped the $ARCH-linux-user and $ARCH-softmmu
builds together (hoping FS cache helps) and grouped all $ARCH-softmmu
only builds into one target. This reduces the build matrix slightly
which will hopefully help with build times.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>