objtool: Don't report end of section error after an empty unwind hint

If asm code specifies an UNWIND_HINT_EMPTY hint, don't warn if the
section ends unexpectedly.  This can happen with the xen-head.S code
because the hypercall_page is "text" but it's all zeros.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Juergen Gross <jgross@suse.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/ddafe199dd8797e40e3c2777373347eba1d65572.1505764066.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

tools/objtool/check.c

@@ -1752,11 +1752,14 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 		if (insn->dead_end)
 			return 0;
 
-		insn = next_insn;
-		if (!insn) {
+		if (!next_insn) {
+			if (state.cfa.base == CFI_UNDEFINED)
+				return 0;
 			WARN("%s: unexpected end of section", sec->name);
 			return 1;
 		}
+
+		insn = next_insn;
 	}
 
 	return 0;