Small cleanup in DWARF reader

I noticed a couple of spots in dwarf/read.c:new_symbol that call
add_symbol_to_list.  However, this function is generally written to
set list_to_add, and then have a single call to add_symbol_to_list at
the end.  This patch cleans up this discrepancy.

Note that new_symbol is overlong and should probably be split up.