objtool: Fix reloc generation on big endian cross-compiles

Relocations generated in elf_rebuild_rel[a]_reloc_section() are broken
if objtool is built and run on a big endian system.

The following errors pop up during x86 cross-compilation:

  x86_64-9.1.0-ld: fs/efivarfs/inode.o: bad reloc symbol index (0x2000000 >= 0x22) for offset 0 in section `.orc_unwind_ip'
  x86_64-9.1.0-ld: final link failed: bad value

Convert those functions to use gelf_update_rel[a](), similar to what
elf_write_reloc() does.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Co-developed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>