MEDIUM: debug: now always print a backtrace on CRASH_NOW() and friends

MEDIUM: debug: now always print a backtrace on CRASH_NOW() and friends

The purpose is to enable the dumping of a backtrace on BUG_ON(). While
it's very useful to know that a condition was met, very often some
caller context is missing to figure how the condition could happen.
From now on, on systems featuring backtrace, a backtrace of the calling
thread will also be dumped to stderr in addition to the unexpected
condition. This will help users of DEBUG_STRICT as they'll most often
find this backtrace in their logs even if they can't find their core
file.

A new "debug dev bug" expert-mode CLI command was added to test the
feature.