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/mips/Kconfig.debug

@@ -1,12 +1,9 @@
 # SPDX-License-Identifier: GPL-2.0
-menu "Kernel hacking"
 
 config TRACE_IRQFLAGS_SUPPORT
 	bool
 	default y
 
-source "lib/Kconfig.debug"
-
 config EARLY_PRINTK
 	bool "Early printk" if EXPERT
 	depends on SYS_HAS_EARLY_PRINTK
@@ -155,5 +152,3 @@ config MIPS_CPS_NS16550_SHIFT
 	  adjacent ns16550 registers in the system.
 
 endif # MIPS_CPS_NS16550_BOOL
-
-endmenu