elf.5: add readelf cross-reference

objdump is sometimes used in cases where readelf is more appropriate,
but the obsolete GNU objdump we have in the base system will be removed
in the future.

.Xr readelf from elf.5 to improve the odds the more appropriate tool
will be found.

PR: 229046
MFC after: 1 week
Sponsored by: The FreeBSD Foundation