We already limit the scope of the cross system build to reduce the
cross build times. With the recent addition of more targets we are
also running into timeout issues for some of the cross user builds.

I've selected a few of those linux-user targets which are less likely
to be in common use as distros don't have pre-built rootfs for them.
I've also added the same CROSS_SKIP_TARGETS variable as is
occasionally used to further limit cross system builds.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220914155950.804707-2-alex.bennee@linaro.org>