libdw: Try .dwp file in __libdw_find_split_unit()

libdw: Try .dwp file in __libdw_find_split_unit()

Try opening the file in the location suggested by the standard (the
skeleton file name + ".dwp") and looking up the unit in the package
index.  The rest is similar to .dwo files, with slightly different
cleanup since a single Dwarf handle is shared.

	* libdw/libdw_find_split_unit.c (try_dwp_file): New function.
	(__libdw_find_split_unit): Call try_dwp_file.
	* libdw/libdwP.h (Dwarf): Add dwp_dwarf and dwp_fd.
	(__libdw_dwp_findcu_id): New declaration.
	(__libdw_link_skel_split): Handle .debug_addr for dwp.
	* libdw/libdw_begin_elf.c (dwarf_begin_elf): Initialize
	result->dwp_fd.
	* libdw/dwarf_end.c (dwarf_end): Free dwarf->dwp_dwarf and close
	dwarf->dwp_fd.
	(cu_free): Don't free split dbg if it is dwp_dwarf.

Signed-off-by: Omar Sandoval <osandov@fb.com>