objtool: Fix 64-bit build on 32-bit host

The new ORC unwinder breaks the build of a 64-bit kernel on a 32-bit
host.  Building the kernel on a i386 or x32 host fails with:

  orc_dump.c: In function 'orc_dump':
  orc_dump.c:105:26: error: passing argument 2 of 'elf_getshdrnum' from incompatible pointer type [-Werror=incompatible-pointer-types]
    if (elf_getshdrnum(elf, &nr_sections)) {
                            ^
  In file included from /usr/local/include/gelf.h:32:0,
                   from elf.h:22,
                   from warn.h:26,
                   from orc_dump.c:20:
  /usr/local/include/libelf.h:304:12: note: expected 'size_t * {aka unsigned int *}' but argument is of type 'long unsigned int *'
   extern int elf_getshdrnum (Elf *__elf, size_t *__dst);
              ^~~~~~~~~~~~~~
  orc_dump.c:190:17: error: format '%lx' expects argument of type 'long unsigned int', but argument 3 has type 'Elf64_Sxword {aka long long int}' [-Werror=format=]
      printf("%s+%lx:", name, rela.r_addend);
        ...