kernel/signal: Make DEBUG_THREAD non-deferrable

Add `SIGNAL_DEBUG_THREAD` to the list of non-deferrable signals.
This allows debuggers to immediately install themselves and break
newly-forked children, as `libroot` requests signals to be deferred
around `_kern_fork`.

Change-Id: Ifc98c3ce4d394f187070a089a9be9fe665ee81d9
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7796
Reviewed-by: waddlesplash <[email protected]>
Tested-by: Commit checker robot <[email protected]>

src/system/kernel/signal.cpp

@@ -63,6 +63,7 @@
 	| SIGNAL_RANGE_TO_MASK(SIGNAL_REALTIME_MIN, SIGNAL_REALTIME_MAX))
 #define NON_DEFERRABLE_SIGNALS	\
 	(KILL_SIGNALS				\
+	| SIGNAL_TO_MASK(SIGNAL_DEBUG_THREAD) \
 	| SIGNAL_TO_MASK(SIGILL)	\
 	| SIGNAL_TO_MASK(SIGFPE)	\
 	| SIGNAL_TO_MASK(SIGSEGV))