Now that __initmv references the machvec section unconditionally
there have been cases where folks have been mistakenly flagging
non-machvec structures with the machvec section attribute (presumably
to shut up modpost). This leads to obscure breakage in earlyprintk
amongst other places and is rather non-obvious.

Add a simple sanity check to try and catch __initmv misuse and
panic early.
Reported-by: Markus Brunner <super.firetwister@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>