objtool: Check local label in read_unwind_hints()

When update the latest upstream gcc and binutils, it generates some
objtool warnings on LoongArch, like this:

  arch/loongarch/kernel/entry.o: warning: objtool: ret_from_fork+0x0: unreachable instruction

We can see that the reloc sym name is local label instead of section
in relocation section '.rela.discard.unwind_hints', in this case, the
reloc sym type is STT_NOTYPE instead of STT_SECTION. Let us check it
to not return -1, then use reloc->sym->offset instead of reloc addend
which is 0 to find the corresponding instruction.

Here are some detailed info:
[fedora@linux 6.8.test]$ gcc --version
gcc (GCC) 14.0.1 20240129 (experimental)
[fedora@linux 6.8.test]$ as --version
GNU assembler (GNU Binutils) 2.42.50.20240129
[fedora@linux 6.8.test]$ readelf -r arch/loongarch/kernel/entry.o | grep -A 3 "rela.discard.unwind_hints"
Relocation section '.rela.discard.unwind_hints' at offset 0x3a8 contains 7 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000000000  000a00000063 R_LARCH_32_PCREL  0000000000000000 .Lhere_1 + 0
00000000000c  000b00000063 R_LARCH_32_PCREL  00000000000000a8 .Lhere_50 + 0

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

Author: seehearfeel

tools/objtool/check.c

@@ -2227,6 +2227,7 @@ static int read_unwind_hints(struct objtool_file *file)
 	struct unwind_hint *hint;
 	struct instruction *insn;
 	struct reloc *reloc;
+	unsigned long offset;
 	int i;
 
 	sec = find_section_by_name(file->elf, ".discard.unwind_hints");
@@ -2254,7 +2255,16 @@ static int read_unwind_hints(struct objtool_file *file)
 			return -1;
 		}
 
-		insn = find_insn(file, reloc->sym->sec, reloc_addend(reloc));
+		if (reloc->sym->type == STT_SECTION) {
+			offset = reloc_addend(reloc);
+		} else if (reloc->sym->local_label) {
+			offset = reloc->sym->offset;
+		} else {
+			WARN("unexpected relocation symbol type in %s", sec->rsec->name);
+			return -1;
+		}
+
+		insn = find_insn(file, reloc->sym->sec, offset);
 		if (!insn) {
 			WARN("can't find insn for unwind_hints[%d]", i);
 			return -1;