powerpc: Add configurable -Werror for arch/powerpc

Add the option to build the code under arch/powerpc with -Werror.

The intention is to make it harder for people to inadvertantly introduce
warnings in the arch/powerpc code. It needs to be configurable so that
if a warning is introduced, people can easily work around it while it's
being fixed.

The option is a negative, ie. don't enable -Werror, so that it will be
turned on for allyes and allmodconfig builds.

The default is n, in the hope that developers will build with -Werror,
that will probably lead to some build breaks, I am prepared to be flamed.

It's not enabled for math-emu, which is a steaming pile of warnings.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/Kconfig.debug

@@ -2,6 +2,23 @@ menu "Kernel hacking"
 
 source "lib/Kconfig.debug"
 
+config PPC_DISABLE_WERROR
+	bool "Don't build arch/powerpc code with -Werror"
+	default n
+	help
+	  This option tells the compiler NOT to build the code under
+	  arch/powerpc with the -Werror flag (which means warnings
+	  are treated as errors).
+
+	  Only enable this if you are hitting a build failure in the
+	  arch/powerpc code caused by a warning, and you don't feel
+	  inclined to fix it.
+
+config PPC_WERROR
+	bool
+	depends on !PPC_DISABLE_WERROR
+	default y
+
 config PRINT_STACK_DEPTH
 	int "Stack depth to print" if DEBUG_KERNEL
 	default 64