objtool: Warn when disabling unreachable warnings

Print a warning when disabling the unreachable warnings (due to a GCC
bug).  This will help determine if recent GCCs still have the issue and
alert us if any other issues might be silently lurking behind the
unreachable disablement.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/df243063787596e6031367e6659e7e43409d6c6d.1742852846.git.jpoimboe@kernel.org

Author: jpoimboe

tools/objtool/arch/x86/special.c

@@ -3,6 +3,7 @@
 
 #include <objtool/special.h>
 #include <objtool/builtin.h>
+#include <objtool/warn.h>
 
 #define X86_FEATURE_POPCNT (4 * 32 + 23)
 #define X86_FEATURE_SMAP   (9 * 32 + 20)
@@ -156,8 +157,10 @@ struct reloc *arch_find_switch_table(struct objtool_file *file,
 	 * indicates a rare GCC quirk/bug which can leave dead
 	 * code behind.
 	 */
-	if (reloc_type(text_reloc) == R_X86_64_PC32)
+	if (reloc_type(text_reloc) == R_X86_64_PC32) {
+		WARN_INSN(insn, "ignoring unreachables due to jump table quirk");
 		file->ignore_unreachables = true;
+	}
 
 	*table_size = 0;
 	return rodata_reloc;