Apply only the libadd/libnames part of dbf05458e3bd

Add libz to the dependencies for libdwarf. Otherwise, building older
branches on recent -CURRENT might fail in the cross-tools stage with:

ld: error: undefined symbol: uncompress

referenced by libdwarf_elf_init.c:233 (/usr/src/contrib/elftoolchain/libdwarf/libdwarf_elf_init.c:233)

libdwarf_elf_init.o:(_dwarf_elf_init) in archive /usr/lib/libdwarf.a

Direct commit to stable/13, since dbf05458e3bd has other libdwarf
changes to support compressed ELF sections, which are more intrusive.