libdw: Make libdw_findcu thread-safe

libdw: Make libdw_findcu thread-safe

        * libdw/libdw_findcu.c (findcu_cb): Use eu_tsearch.
        (__libdw_findcu): Use eu_tfind and dwarf_lock
        (__libdw_findcu_addr): Use eu_tfind.
        (__libdw_find_split_dbg_addr): Likewise.

Signed-off-by: Heather S. McIntyre <hsm2@rice.edu>
Signed-off-by: Aaron Merey <amerey@redhat.com>
Signed-off-by: Mark Wielaard <mark@klomp.org>
v2 changes:
Use per-Dwarf_CU lock instead of a global lock.