Add check for missing wrapper headers

Add check for missing wrapper headers

If building on a subset of architectures only, it is easy to miss
wrapper headers which are required by other architectures because
they lack the corresponding sysdeps header.  The check ensures
that every installed header which is not itself a sysdeps header
has a header under include/ (that presumably wraps the header,
and perhaps also adding declarations and definitions for !_ISOMAC).

Also check for the absence of the sysdeps/generic/bits directory
removed in commit c72565e5f1124c2dc72573e83406fe999e56091f, to make
accidental re-introduction more difficult.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>