addr2line: add label checks when DW_AT_range and DW_AT_low_pc cannot be used

Check label's ranges for address we want to translate if a CU doesn't
have usable DW_AT_range or DW_AT_low_pc.

Use more appropriate names: "struct CU" -> "struct range"

Developed as part of upstream ELF Tool Chain bug report
https://sourceforge.net/p/elftoolchain/tickets/552/ although this does
not address the specific case reported there.

Submitted by: Tiger Gao <tig@freebsdfoundation.org>
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23782