MINOR: debug: extract the backtrace dumping code to its own function

MINOR: debug: extract the backtrace dumping code to its own function

The backtrace dumping code was located into the thread dump function
but it looks particularly convenient to be able to call it to produce
a dump in other situations, so let's move it to its own function and
make sure it's called last in the function so that we can benefit from
tail merging to save one entry.