Kconfig: consolidate the "Kernel hacking" menu

Move the source of lib/Kconfig.debug and arch/$(ARCH)/Kconfig.debug to
the top-level Kconfig.  For two architectures that means moving their
arch-specific symbols in that menu into a new arch Kconfig.debug file,
and for a few more creating a dummy file so that we can include it
unconditionally.

Also move the actual 'Kernel hacking' menu to lib/Kconfig.debug, where
it belongs.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

arch/nios2/Kconfig.debug

@@ -1,11 +1,8 @@
 # SPDX-License-Identifier: GPL-2.0
-menu "Kernel hacking"
 
 config TRACE_IRQFLAGS_SUPPORT
 	def_bool y
 
-source "lib/Kconfig.debug"
-
 config DEBUG_STACK_USAGE
 	bool "Enable stack utilization instrumentation"
 	depends on DEBUG_KERNEL
@@ -24,5 +21,3 @@ config EARLY_PRINTK
 	  This is useful for kernel debugging when your machine crashes very
 	  early before the console code is initialized.
 	  You should normally say N here, unless you want to debug such a crash.
-
-endmenu