gdb: add target_debug_printf and target_debug_printf_nofunc

gdb: add target_debug_printf and target_debug_printf_nofunc

Add the `target_debug_printf` and `target_debug_printf_nofunc` macros
and use them when outputting debug messages depending on `targetdebug`.
I opted for `target_debug_printf_nofunc` to follow the current style
where the function name is already printed, along with the arguments.

Modify the debug printfs in the `debug_target` methods (generated by
`make-target-delegates.py`) to use `target_debug_printf_nofunc` as well.

This makes the "target" debug prints integrate nicely with the other
debug prints that use the "new" debug print system:

    [infrun] proceed: enter
      [infrun] follow_fork: enter
        [target] -> multi-thread->record_will_replay (...)
        [target] <- multi-thread->record_will_replay (-1, 0) = false
        [target] -> multi-thread->supports_multi_process (...)
        [target] <- multi-thread->supports_multi_process () = true
      [infrun] follow_fork: exit
      ...

Change-Id: Ide3c8c1b8a30e6d4c353a29cba911c7192de29ac
Approved-By: Tom Tromey <tom@tromey.com>