Support for HWbreak/watchpoint across fork/vfork on arm-native

This patch updates arm native support for hwbreak-/watchpoints to enable
support for hwbreak-/watchpoints across fork/vfork. This involves changes to
hwbreak-/watchpoint insertion mechanism to the modern way, by marking debug
registers as needing update, but only really updating them on resume, which is
necessary for supporting watchpoints in non-stop mode. This also updates a
previously maintained per thread hwbreak-/watchpoint cache to a per process
cache which allows target specific code to come in sync with gdb-linux calls to
threads create/destroy and process fork/exit hooks.
Factor out foreground/background execution command preparation.

All execution commands currently have this pattern:

  /* If we must run in the background, but the target can't do it,
     error out.  */
  if (async_exec && !target_can_async_p ())
    error (_("Asynchronous execution not supported on this target."));

  /* If we are not asked to run in the bg, then prepare to run in the
     foreground, synchronously.  */
  if (!async_exec && target_can_async_p ())
    {
      /* Simulate synchronous execution.  */
      async_disable_stdin ();
    }

This patch factors that into a shared function.

attach_command installs a cleanup to re-enable stdin, but that's not
necessary, as per the comment in prepare_execution_command.  In any
case, if someday it turns out necessary, we have a single place to
install it now.

Tested on x86_64 Fedora 17, sync and async modes.

gdb/
2014-03-12  Pedro Alves  <palves@redhat.com>

	* infcmd.c (prepare_execution_command): New function, factored out
	from several execution commands.
	(run_command_1, continue_command, step_1, jump_command)
	(signal_command, until_command, advance_command, finish_command)
	(attach_command): Use prepare_execution_command.