rts: Fix `prompt#` when profiling is enabled

This MR fixes issue #23001 (closed). The bug was really just the result of a major oversight on my part, namely never actually testing with -prof! The patch enables testing with -prof to the tests, so this should not cause any trouble in the future.

In addition to the fix (which is quite small), this patch also adds a -Dk RTS option to the debug RTS, which prints out some debugging information on continuation capture. Currently, the printed information is quite minimal, but it can be extended in the future if more things prove to be useful to future debugging efforts.