Support Linux boot protocol on AArch64

[mkroening]

In 6800f4e and 6897ae8, @fstracke added initial stubs for the Linux boot protocol on AArch64.

This has been reverted in #543, since more work is required to make this work properly with U-Boot's booti: 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 for details.