hardening: Enable i386 FORTIFY_SOURCE on Clang 16+

The i386 regparm bug exposed with FORTIFY_SOURCE with Clang was fixed
in Clang 16[1].

Link: llvm/llvm-project@c167c0a [1]
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20250308042929.1753543-2-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

Author: kees

arch/x86/Makefile

@@ -137,7 +137,7 @@ ifeq ($(CONFIG_X86_32),y)
         include $(srctree)/arch/x86/Makefile_32.cpu
         KBUILD_CFLAGS += $(cflags-y)
 
-    ifeq ($(CONFIG_CC_IS_CLANG),y)
+    ifneq ($(call clang-min-version, 160000),y)
         # https://github.com/llvm/llvm-project/issues/53645
         KBUILD_CFLAGS += -ffreestanding
     endif

security/Kconfig.hardening

@@ -286,7 +286,7 @@ config FORTIFY_SOURCE
 	bool "Harden common str/mem functions against buffer overflows"
 	depends on ARCH_HAS_FORTIFY_SOURCE
 	# https://github.com/llvm/llvm-project/issues/53645
-	depends on !CC_IS_CLANG || !X86_32
+	depends on !X86_32 || !CC_IS_CLANG || CLANG_VERSION >= 160000
 	help
 	  Detect overflows of buffers in common string and memory functions
 	  where the compiler can determine and validate the buffer sizes.