Optional fall back to LLVM symbolizer markup

Now that Symbolizer Markup has landed in LLVM 15.0.2, it would be possible to add an option to emit this markup whenever symbol information is unavailable (fully-stripped binaries, etc.).

Producing symbolizer markup would allow llvm-symbolizer to produce human-readable backtraces from it after the fact, looking up the necessary artifacts using debuginfod. This complements https://github.com/rust-lang/backtrace-rs/issues/459; since it would be nice to have debuginfod lookups handled transparently, but that's not always possible (e.g., for embedded or sandboxed environments).

This should be relatively straightforward to implement, since the format is just a simple textual representation of the memory layout of the process and the return addresses of the backtrace.

Start with LLVM's Symbolizer Markup Format and the existing backtrace output paths in backtrace-rs; the issue does not name specific files or tests. Determine where symbol information is unavailable and how an option should select markup output. Done means fully stripped or otherwise unsymbolized backtraces can emit markup that llvm-symbolizer can later resolve using debuginfod.