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/nds32/Kconfig

@@ -91,7 +91,3 @@ menu "Kernel Features"
 source "kernel/Kconfig.preempt"
 source "kernel/Kconfig.hz"
 endmenu
-
-menu "Kernel hacking"
-source "lib/Kconfig.debug"
-endmenu

arch/nds32/Kconfig.debug

@@ -0,0 +1 @@
+# dummy file, do not delete