efi/libstub: Disable Shadow Call Stack

Shadow stacks are not available in the EFI stub, filter out SCS flags.
Suggested-by: NJames Morse <james.morse@arm.com>
Signed-off-by: NSami Tolvanen <samitolvanen@google.com>
Reviewed-by: NKees Cook <keescook@chromium.org>
Acked-by: NArd Biesheuvel <ardb@kernel.org>
Signed-off-by: NWill Deacon <will@kernel.org>

drivers/firmware/efi/libstub/Makefile

@@ -32,6 +32,9 @@ KBUILD_CFLAGS			:= $(cflags-y) -DDISABLE_BRANCH_PROFILING \
 				   $(call cc-option,-fno-stack-protector) \
 				   -D__DISABLE_EXPORTS
 
+# remove SCS flags from all objects in this directory
+KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_SCS), $(KBUILD_CFLAGS))
+
 GCOV_PROFILE			:= n
 KASAN_SANITIZE			:= n
 UBSAN_SANITIZE			:= n