Backtrace support

[abrestic-rivos]

It would be helpful if we could dump a backtrace when Salus panics. Doing this in no_std gets a bit tricky, though.

From some quick searching I've found mini-backtrace, which uses libunwind. Only PCs though, no symbol names. There's also backtracer, which looks like it does what we want, but has x86 dependencies and very little usage.