BFD: Add BFD pointer member to `struct carsym'

Replace the file offset with a union of one and a BFD pointer in `struct
carsym', as a preparation to handle symbol maps built on the fly rather
than fetched from a file.  No functional change.

NB there's a note on `struct carsym' being a type pun on `struct symdef'
as in include/aout/ranlib.h (or `struct ranlib'; no idea where it comes
from).  It is possibly not true anymore, even more so with this code
update in place, depending on the underlying type of `file_ptr'.  This
is not a problem however, because we always process `struct carsym' data
by hand and only use it internally rather than reading/writing from/to a
file.  Therefore remove the note, as no longer applicable.