powerpc/vdso: Filter clang's auto var init zero enabler when linking

After commit 8d9acfce ("kbuild: Stop using '-Qunused-arguments' with
clang"), the PowerPC vDSO shows the following error with clang-13 and
older when CONFIG_INIT_STACK_ALL_ZERO is enabled:

  clang: error: argument unused during compilation: '-enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang' [-Werror,-Wunused-command-line-argument]

clang-14 added a change to make sure this flag never triggers
-Wunused-command-line-argument, so it is fixed with newer releases. For
older releases that the kernel still supports building with, just filter
out this flag, as has been done for other flags.

Fixes: f0a42fba ("powerpc/vdso: Improve linker flags")
Fixes: 8d9acfce ("kbuild: Stop using '-Qunused-arguments' with clang")
Link: https://github.com/llvm/llvm-project/commit/ca6d5813d17598cd180995fb3bdfca00f364475fSigned-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -912,7 +912,9 @@ ifdef CONFIG_INIT_STACK_ALL_ZERO
 KBUILD_CFLAGS	+= -ftrivial-auto-var-init=zero
 ifdef CONFIG_CC_HAS_AUTO_VAR_INIT_ZERO_ENABLER
 # https://github.com/llvm/llvm-project/issues/44842
-KBUILD_CFLAGS	+= -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
+CC_AUTO_VAR_INIT_ZERO_ENABLER := -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
+export CC_AUTO_VAR_INIT_ZERO_ENABLER
+KBUILD_CFLAGS	+= $(CC_AUTO_VAR_INIT_ZERO_ENABLER)
 endif
 endif
 

arch/powerpc/kernel/vdso/Makefile

@@ -56,7 +56,7 @@ ccflags-y := -fno-common -fno-builtin
 ldflags-y := -Wl,--hash-style=both -nostdlib -shared -z noexecstack
 ldflags-$(CONFIG_LD_IS_LLD) += $(call cc-option,--ld-path=$(LD),-fuse-ld=lld)
 # Filter flags that clang will warn are unused for linking
-ldflags-y += $(filter-out $(CC_FLAGS_FTRACE) -Wa$(comma)%, $(KBUILD_CFLAGS))
+ldflags-y += $(filter-out $(CC_AUTO_VAR_INIT_ZERO_ENABLER) $(CC_FLAGS_FTRACE) -Wa$(comma)%, $(KBUILD_CFLAGS))
 
 CC32FLAGS := -m32
 LD32FLAGS := -Wl,-soname=linux-vdso32.so.1