[gdb/symtab] Use comp_unit_head::get_length

There's a spot in read_comp_units_from_section where we explictly use
initial_length_size to get the total length:
...
      this_cu->length = cu_header.length + cu_header.initial_length_size;
...

Instead, just use cu_header.get_length ().

Tested on x86_64-linux.