objtool: Enable unreachable warnings for CLANG LTO

With IBT support in, objtool is now fully capable of following vmlinux
code flow in LTO mode.  Start reporting unreachable warnings for Clang
LTO as well.

Fixes: ed53a0d ("x86/alternative: Use .ibt_endbr_seal to seal indirect calls")
Signed-off-by: Josh Poimboeuf <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/7b12df54bceeb0761fe9fc8269ea0c00501214a9.1650300597.git.jpoimboe@redhat.com

Author: jpoimboe

scripts/Makefile.build

@@ -231,7 +231,7 @@ objtool_args =								\
 	$(if $(part-of-module), --module)				\
 	$(if $(CONFIG_X86_KERNEL_IBT), --lto --ibt)			\
 	$(if $(CONFIG_FRAME_POINTER),, --no-fp)				\
-	$(if $(CONFIG_GCOV_KERNEL)$(CONFIG_LTO_CLANG), --no-unreachable)\
+	$(if $(CONFIG_GCOV_KERNEL), --no-unreachable)			\
 	$(if $(CONFIG_RETPOLINE), --retpoline)				\
 	$(if $(CONFIG_X86_SMAP), --uaccess)				\
 	$(if $(CONFIG_FTRACE_MCOUNT_USE_OBJTOOL), --mcount)		\

scripts/link-vmlinux.sh

@@ -140,7 +140,7 @@ objtool_link()
 		if ! is_enabled CONFIG_FRAME_POINTER; then
 			objtoolopt="${objtoolopt} --no-fp"
 		fi
-		if is_enabled CONFIG_GCOV_KERNEL || is_enabled CONFIG_LTO_CLANG; then
+		if is_enabled CONFIG_GCOV_KERNEL; then
 			objtoolopt="${objtoolopt} --no-unreachable"
 		fi
 		if is_enabled CONFIG_RETPOLINE; then