checkpatch: try to catch missing VMLINUX_SYMBOL() in vmlinux.lds.h

Seems like every other release we have someone who updates vmlinux.lds.h
and adds C-visible symbols without VMLINUX_SYMBOL() around them.  So start
checking the file and reject assignments which have plain symbols on
either side.

[apw@canonical.com: soften the check, add tests]
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>