Avoid race when reading dwz file

Avoid race when reading dwz file

PR gdb/31260 points out a race introduced by the background reading
changes.  If a given objfile is re-opened when it is already being
read, dwarf2_initialize_objfile will call dwarf2_read_dwz_file again,
causing the 'dwz_file' to be reset.

This patch fixes the problem by arranging to open the dwz just once:
when the dwarf2_per_bfd object is created.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31260