gdb/dwarf: pass unit length to dwarf2_per_cu_data constructor

Most of the time, the length of a unit is known when constructing a
dwarf2_per_cu_data or signatured_type.  Add a construtor parameter for
it.  In the few cases where it isn't, pass 0, which leaves it unset.

Change-Id: I0c8b9683164d3e3f3823ed995f71689a4d17fd96
Reviewed-By: Tom de Vries <tdevries@suse.de>