lib/syscall_shim/arch/arm64: Store caller's link register

lib/syscall_shim/arch/arm64: Store caller's link register

So far for storing LR and ELR_EL1 we have been using the current
link register for both instead. This does not reflect reality when
it comes to LR as we should instead store the LR that the caller
would know. To achieve this, do a brief level 1 stack unwinding and
fetch caller's LR from the top of its frame.

Signed-off-by: Sergiu Moga <sergiu@unikraft.io>
Approved-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
GitHub-Closes: #1618