powerpc/kcsan: Add KCSAN Support

Enable HAVE_ARCH_KCSAN for 64-bit Book3S, permitting use of the kernel
concurrency sanitiser through the CONFIG_KCSAN_* kconfig options. KCSAN
requires compiler builtins __atomic_* 64-bit values, and so only report
support on 64-bit.

See documentation in Documentation/dev-tools/kcsan.rst for more
information.

Signed-off-by: Rohan McLure <rmclure@linux.ibm.com>
[mpe: Limit to Book3S to avoid build failure on Book3E]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230206021801.105268-6-rmclure@linux.ibm.com

arch/powerpc/Kconfig

@@ -198,6 +198,7 @@ config PPC
 	select HAVE_ARCH_KASAN			if PPC_RADIX_MMU
 	select HAVE_ARCH_KASAN			if PPC_BOOK3E_64
 	select HAVE_ARCH_KASAN_VMALLOC		if HAVE_ARCH_KASAN
+	select HAVE_ARCH_KCSAN            	if PPC_BOOK3S_64
 	select HAVE_ARCH_KFENCE			if ARCH_SUPPORTS_DEBUG_PAGEALLOC
 	select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
 	select HAVE_ARCH_KGDB