libdw: Make libdw_find_split_unit thread-safe

libdw: Make libdw_find_split_unit thread-safe

	* libdw/dwarf_end.c (cu_free): Free split_lock.
	* libdw/dwarf_formref_die.c (dwarf_formref_die): Add locking
	  around call to __libdw_intern_next_unit.
	* libdw/libdwP.h (struct Dwarf_CU): Add split_lock.
	* libdw/libdw_find_split_unit.c (__libdw_find_split_unit):
	  Add locking.
	* libdw/libdw_findcu.c (__libdw_intern_next_unit): Init
	  split_lock.

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>