Stream DWARF debug section directly to disk.

Change the structure so that the debug sections are written to
disk as we go.  There are still some temporary buffers, however,
we no longer hold all of the data in memory before writing it.
We can not avoid buffering of some things (e.g. .debug_str).

Change-Id: Id4940cf10ae5b6f3cac7fb8d20197f0304079b92