skip fixinc on vxworks7*, amend mkheaders

vxworks7 headers haven't required fixes, and we've decided to avoid
running fixinc on them.

The problem with that is that, with a dummy fixinc, mkheaders wipes
out include-fixed but then multi_dir subdirs are not created again, so
we end up with a limits.h named after each multi_dir, when there are
non-default multilibs.  Oops.

This patch arranges for a dummy fixinc to be created for *-*-vxworks7*
targets, and fixes mkheaders so as to create multi_dir subdirs in
include-fixed after wiping them out, and to copy limits.h so that it
won't take the name that should be of a subdir (unless the multi_dir
is limits.h, but that's hopefully never the case ;-)


for  fixincludes/ChangeLog

	* mkheaders.in: Re-create subdirs, copy limits.h into subdir.
	* mkfixinc.sh: Create dummy fixinc for *-*-vxworks7*.