objtool: Upgrade libelf-devel warning to error for CONFIG_ORC_UNWINDER

With CONFIG_ORC_UNWINDER, if the user doesn't have libelf-devel
installed, and they don't see the make warning, their ORC unwinder will
be silently broken.  Upgrade the warning to an error.

Reported-and-tested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/d9dfc39fb8240998820f9efb233d283a1ee96084.1507079417.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

Makefile

@@ -933,7 +933,11 @@ ifdef CONFIG_STACK_VALIDATION
   ifeq ($(has_libelf),1)
     objtool_target := tools/objtool FORCE
   else
-    $(warning "Cannot use CONFIG_STACK_VALIDATION, please install libelf-dev, libelf-devel or elfutils-libelf-devel")
+    ifdef CONFIG_ORC_UNWINDER
+      $(error "Cannot generate ORC metadata for CONFIG_ORC_UNWINDER=y, please install libelf-dev, libelf-devel or elfutils-libelf-devel")
+    else
+      $(warning "Cannot use CONFIG_STACK_VALIDATION=y, please install libelf-dev, libelf-devel or elfutils-libelf-devel")
+    endif
     SKIP_STACK_VALIDATION := 1
     export SKIP_STACK_VALIDATION
   endif