Move DWARF related code in ElfWriter to its own file.

This is purely for organisation.  The DWARF related code
is becoming more complex and I do not want to keep mixing
it with the ElfWriter.  The ElfWriter probably should not
be dealing with the fine details of DWARF generation.

I intend to do much more work on DWARF in the future so
this situatuion would only get worse as the code grows.

The code could also use some refactoring and splitting.
However, I have mostly just moved it in this CL.

Change-Id: I6e3401a2b745ddf991b440ad7899ad7d4b921be6