Stack trace capture for ptrace backend

[kxxt]

We could add a new cli option that enables capture and symbolize stack traces when exec happens.

Or when a breakpoint is hit in TUI mode, here we could add an action to view its stack trace.

[kxxt]

We could possibly use https://docs.rs/rstack/latest/rstack/ but risc-v support is still not there yet: sfackler/rstack#29