Current gdbserver includes gdb_thread_db.h directly from gdb in
gdbserver/thread-db.c like this,

#include "../gdb_thread_db.h"

It is a signal that gdb_thread_db.h should be moved to common/

gdb_thread_db.h is using only one HAVE_* macro checking,
HAVE_THREAD_DB_H, which is checked by configure stuff in both GDB and
GDBserver.  It is safe to move it to common/ dir.

This patch also tracks the dependence of thread-db.o on
common/gdb_thread_db.h.  OK?

-- 
Yao (齐尧)