elf: add Elf64_Sym

Required as Elf_Sym in tools/prelink-riscv.inc. I assume people have
been using an OS-supplied elf.h, but macOS doesn't have that.

Taken from
https://github.com/torvalds/linux/blob/v6.1/include/uapi/linux/elf.h



Signed-off-by: KaDiWa <kalle.wachsmuth@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>