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/alpha/Kconfig.debug

@@ -1,7 +1,4 @@
 # SPDX-License-Identifier: GPL-2.0
-menu "Kernel hacking"
-
-source "lib/Kconfig.debug"
 
 config EARLY_PRINTK
 	bool
@@ -39,5 +36,3 @@ config MATHEMU
 	  This option is required for IEEE compliant floating point arithmetic
 	  on the Alpha. The only time you would ever not say Y is to say M in
 	  order to debug the code. Say Y unless you know what you are doing.
-
-endmenu