[PAC][LLDB][Dwarf] Encode the default ABI signing schema in debug info

Quoting @ojhunt https://github.com/llvm/llvm-project/pull/186001#issuecomment-4080304382:

> It would be nice to just use target triple information, and lldb can certainly do it for ABIs it has direct knowledge of, but having the information about the default target ABI simply embedded in the binary seems like it would be more robust and support varying/evolving linux ABIs as well.

Start by reviewing the linked LLVM pull request and its cited comment, then trace how LLDB consumes DWARF debug information and how target triple information currently determines the ABI. The issue does not name files or tests, so first identify the relevant compiler and LLDB entry points; done means the default ABI signing schema is encoded in debug info and used by LLDB.

Written by the indexing model from the issue text.