gdb/dwarf: allocate dwo_unit with new

gdb/dwarf: allocate dwo_unit with new

The following patch reduces the duration where the dwo_lock mutex is
taken.  One operation that is not thread safe is the allocation on
dwo_units on the per_bfd obstack:

    dwo_unit *dwo_unit = OBSTACK_ZALLOC (&per_bfd->obstack, struct dwo_unit);

We could take the lock around this allocation, but I think it's just
easier to avoid the problem by having the dwo_unit objects allocated
with "new".

Change-Id: Ida04f905cb7941a8826e6078ed25dbcf57674090
Approved-By: Tom Tromey <tom@tromey.com>