arch/arm64: Save/restore link register on `clone` asm stub

arch/arm64: Save/restore link register on `clone` asm stub

Since we are invoking `clone` as a regular function call instead
of a system call, there is nobody to remember the asm stub entry
link register for us once we branch into the syscall. Therefore
save and restore it.

Signed-off-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
GitHub-Closes: #89