Support Linux boot protocol on AArch64

In https://github.com/hermit-os/loader/commit/6800f4e356ab6398c9a3ec51538a95948f7989c8 and https://github.com/hermit-os/loader/commit/6897ae83e95300afbb236a9aa6e351cb6f816405, @fstracke added initial stubs for the Linux boot protocol on AArch64.

This has been reverted in https://github.com/hermit-os/loader/pull/543, since more work is required to make this work properly with [U-Boot's `booti`](https://docs.u-boot.org/en/latest/usage/cmd/booti.html): converting the loader ELF to an appropriate image with `llvm-objcopy` works, but for the image to be runnable, the code would need to be position-independent.

See [`arm64_image_header`](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/include/asm/image.h?h=v6.18) for details.