copy-builtin: SPL must be in Kbuild first

The recent SPL merge caused a regression in kernels with ZFS integrated
into the sources where our modules would be initialized in alphabetical
order, despite icp requiring the spl module be loaded first. This caused
kernels with ZFS builtin to fail to boot.

We resolve this by adding a special case for the spl that lists it
first. It is somewhat ugly, but it works.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Thode <prometheanfire@gentoo.org>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Closes #7595
Closes #7606