MINOR: debug: opportunistically load libthread_db.so.1 with set-dumpable=libs

MINOR: debug: opportunistically load libthread_db.so.1 with set-dumpable=libs

When loading libs into the core dump, let's also try to load
libthread_db.so.1 that gdb usually requires. It can significantly help
decoding the threads for systems which require it, and the file is quite
small. It can appear at a few different locations and is generally next
to libpthread.so, or alternately libc, so we first look where we found
them, and fall back to a few other common places. The file is really
small, a few tens of kB usually.