Change the_dummy_target to be a global

Change the_dummy_target to be a global

While debugging gdb, I printed the target stack and got:

    (top-gdb) p g_target_stack
    $10 = {
      m_top = thread_stratum,
      m_stack = {0x142b0b0, 0x13da600 <exec_ops>, 0x1c70690, 0x13d63b0 <ravenscar_ops>, 0x0, 0x0, 0x0}
    }

(This is clearly from before the change to make ravenscar
multi-target-capable.)

Here, 0x142b0b0 is the singleton dummy target.  It seems to me that
since this is always a singleton, it would be a bit nicer if it were a
global, so that it would be noted in the above.

This patch implements this idea, and now I get:

    (top-gdb) p g_target_stack
    $2 = {
      m_top = dummy_stratum,
      m_stack = {0x1f1b040 <the_dummy_target>, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}
    }

I did not do the same for the debug target.  It didn't seem as useful
to me.

gdb/ChangeLog
2019-03-05  Tom Tromey  <tromey@adacore.com>

	* target.c (the_dummy_target): Move later.  Change type to
	"dummy_target".
	(initialize_targets): Don't initialize the_dummy_target.