[arch][arm64] tweak the arm64_elX_to_el1 routine to avoid using x0-x3
[arch][arm64] tweak the arm64_elX_to_el1 routine to avoid using x0-x3

It's called immediately upon entering the kernel entry vector, prior
to knowing if it's the boot cpu or needing to save any boot arguments,
so avoid using these registers

littlekernel/lk,hollanderic/lkstuff,littlekernel/lk,littlekernel/lk,littlekernel/lk,hollanderic/lkstuff,hollanderic/lkstuff,hollanderic/lkstuff