PR 16329: remote debugging broken on Solaris.

PR 16329: remote debugging broken on Solaris.

Like on GNU/Linux (linux-thread-db.c), the Solaris solaris-threads
target (handles libthread_db.so) shouldn't be pushed when remote
debugging.

This uses the same predicate used by linux-thread-db.c.

gdb/
2013-12-16  Pedro Alves  <palves@redhat.com>

	PR 16329
	* sol-thread.c (check_for_thread_db): If the target can't run or
	isn't a core, return without pushing.