Always print the "host libthread-db" message to stdout

linux-thread-db.c has a bit of unusual code that unconditionally
prints a message, but decides whether to print to gdb_stdout or
gdb_stdlog based on a debug flag.  It seems better to me to simply
always print this; and this is the only spot in gdb where we
conditionally pass gdb_stdout to one of the f*_unfiltered functions.