powerpc: Adjust adding stack protector flags to KBUILD_CLAGS for clang

After fixing the HAVE_STACKPROTECTER checks for clang's in-progress
per-task stack protector support [1], the build fails during prepare0
because '-mstack-protector-guard-offset' has not been added to
KBUILD_CFLAGS yet but the other '-mstack-protector-guard' flags have.

  clang: error: '-mstack-protector-guard=tls' is used without '-mstack-protector-guard-offset', and there is no default
  clang: error: '-mstack-protector-guard=tls' is used without '-mstack-protector-guard-offset', and there is no default
  make[4]: *** [scripts/Makefile.build:229: scripts/mod/empty.o] Error 1
  make[4]: *** [scripts/Makefile.build:102: scripts/mod/devicetable-offsets.s] Error 1

Mirror other architectures and add all '-mstack-protector-guard' flags
to KBUILD_CFLAGS atomically during stack_protector_prepare, which
resolves the issue and allows clang's implementation to fully work with
the kernel.

Cc: stable@vger.kernel.org # 6.1+
Link: llvm/llvm-project#110928 [1]
Reviewed-by: Keith Packard <keithp@keithp.com>
Tested-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/20241009-powerpc-fix-stackprotector-test-clang-v2-2-12fb86b31857@kernel.org

Author: nathanchance

arch/powerpc/Makefile

@@ -100,13 +100,6 @@ KBUILD_AFLAGS	+= -m$(BITS)
 KBUILD_LDFLAGS	+= -m elf$(BITS)$(LDEMULATION)
 endif
 
-cflags-$(CONFIG_STACKPROTECTOR)	+= -mstack-protector-guard=tls
-ifdef CONFIG_PPC64
-cflags-$(CONFIG_STACKPROTECTOR)	+= -mstack-protector-guard-reg=r13
-else
-cflags-$(CONFIG_STACKPROTECTOR)	+= -mstack-protector-guard-reg=r2
-endif
-
 LDFLAGS_vmlinux-y := -Bstatic
 LDFLAGS_vmlinux-$(CONFIG_RELOCATABLE) := -pie
 LDFLAGS_vmlinux-$(CONFIG_RELOCATABLE) += -z notext
@@ -402,9 +395,11 @@ prepare: stack_protector_prepare
 PHONY += stack_protector_prepare
 stack_protector_prepare: prepare0
 ifdef CONFIG_PPC64
-	$(eval KBUILD_CFLAGS += -mstack-protector-guard-offset=$(shell awk '{if ($$2 == "PACA_CANARY") print $$3;}' include/generated/asm-offsets.h))
+	$(eval KBUILD_CFLAGS += -mstack-protector-guard=tls -mstack-protector-guard-reg=r13 \
+				-mstack-protector-guard-offset=$(shell awk '{if ($$2 == "PACA_CANARY") print $$3;}' include/generated/asm-offsets.h))
 else
-	$(eval KBUILD_CFLAGS += -mstack-protector-guard-offset=$(shell awk '{if ($$2 == "TASK_CANARY") print $$3;}' include/generated/asm-offsets.h))
+	$(eval KBUILD_CFLAGS += -mstack-protector-guard=tls -mstack-protector-guard-reg=r2 \
+				-mstack-protector-guard-offset=$(shell awk '{if ($$2 == "TASK_CANARY") print $$3;}' include/generated/asm-offsets.h))
 endif
 endif