gdb: Don't call gdb_load_shlib unless GDB is running

gdb: Don't call gdb_load_shlib unless GDB is running

The gdb_load_shlib function will, on remote targets, try to run some
GDB commands.  This obviously isn't going to work unless GDB is
running.

The gdb.trace/tspeed.exp test calls gdb_load_shlib before starting
GDB.  Don't do that.

The failure that's triggered is actually DeJaGNU complaining that the
variable $use_gdb_stub doesn't exist, this is only created when GDB is
started.  Something like this should trigger a failure:

  make check-gdb \
    RUNTESTFLAGS="--target_board=remote-gdbserver-on-localhost \
                  gdb.trace/tspeed.exp"

This commit also adds a check to gdb_load_shlib that GDB is running.
The check is always performed, so this should catch cases where a GDB
developer adds a use of gdb_load_shlib but doesn't test their code
with a remote target.

gdb/testsuite/ChangeLog:

	* gdb.trace/tspeed.exp: Only call gdb_load_shlib after gdb has
	started.
	* lib/gdb.exp (gdb_load_shlib): Call perror if GDB is not running.