kbuild: lto: force rebuilds when switching CONFIG_LTO

When doing non-clean builds and switching between CONFIG_LTO=n and
CONFIG_LTO=y, the build system (correctly) didn't notice that assembly
and LTO-excluded C object files were rewritten in place by objtool (to
add the .orc_unwind* sections), since their build command lines were the
same between CONFIG_LTO=y and CONFIG_LTO=n. The objtool step would fail:

vmlinux.o: warning: objtool: file already has .orc_unwind section, skipping
make: *** [Makefile:1194: vmlinux] Error 255

Avoid this by making sure the build will see a difference between an LTO
and non-LTO build (by including "-fno-lto" in KBUILD_*FLAGS). This will
get ignored when CC_FLAGS_LTO is present, and will not be included at
all when CONFIG_LTO=n.
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

Makefile

@@ -910,7 +910,8 @@ KBUILD_LDFLAGS += -mllvm -import-instr-limit=5
 endif
 
 ifdef CONFIG_LTO
-KBUILD_CFLAGS	+= $(CC_FLAGS_LTO)
+KBUILD_CFLAGS	+= -fno-lto $(CC_FLAGS_LTO)
+KBUILD_AFLAGS	+= -fno-lto
 export CC_FLAGS_LTO
 endif