[docs][llvm-nm][llvm-objdump] Improve "See Also" section

The "See Also" section for llvm-nm didn't actually contain any links,
and the tools referred to didn't make much sense (referring to non-LLVM
tools, when we have equivalents, or tools that aren't really to do with
symbol dumping). llvm-objdump's didn't refer to llvm-readelf.

Reviewed by: grimar

Differential Revision: https://reviews.llvm.org/D63875

llvm-svn: 364552