gdb/dwarf: add unit_lists structure to index writer

I think it makes the code more readable than the pair of vector.  Also,
I'm considering adding a third list (foreigh type units), which will be
easier with the structure.

Change-Id: I38ec4ddf8f786a2ba10c5b371cfe04c2baaa7da9
Approved-By: Tom Tromey <tom@tromey.com>