libdw: Make sure all attributes come with a (fake) CU for bound checks.

libdw: Make sure all attributes come with a (fake) CU for bound checks.

All attributes now have a reference to a (fake) CU that has startp and
endp set to the data section where the form data comes from. Use that
for bounds checking in __libdw_form_val_len and dwarf_formblock to make
sure data read doesn't overflow any data section. Remove libdwP.h cu_data
and use cu startp and endp directly where appropriate.

Signed-off-by: Mark Wielaard <mjw@redhat.com>