SA: Remove libthread_db dependency on Linux

For live (attach to a process) debugging on Linux, SA depends on the libthread_db library predominantly to get the list of existing threads and the mapping between the user thread ids and the lwpids (which is subsequently passed to the ptrace calls). libthread_db predates Linux's NPTL, and the relevance of this seems questionable given that the entire list of threads can now be obtained by reading in the contents of the /proc/<pid>/task directory on Linux.

The code depending on thread_db can most likely be removed and /proc can be used instead.