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/arc/Kconfig.debug

@@ -1,7 +1,4 @@
 # SPDX-License-Identifier: GPL-2.0
-menu "Kernel hacking"
-
-source "lib/Kconfig.debug"
 
 config 16KSTACKS
 	bool "Use 16Kb for kernel stacks instead of 8Kb"
@@ -11,5 +8,3 @@ config 16KSTACKS
 	  This increases the resident kernel footprint and will cause less
 	  threads to run on the system and also increase the pressure
 	  on the VM subsystem for higher order allocations.
-
-endmenu