[lldb] Add extra logic to find llvm-nm in standalone builds (#227487)

If one builds LLDB standalone against an LLVM that builds but does not
export an llvm-nm target, then LLDB will fail to build when also using
dynamic script interpreters.

To account for this, we use the same logic for finding llvm-tblgen. I
intentionally did not deduplicate the logic because they are run in
slightly different scenarios. I plan on joining these together with a
macro once another tool is needed.

GitOrigin-RevId: 65d6d9c8a189c3e6623fe974df5fe623bdde9f7a