patches: Re-add `UnwindRegistersSave.S` RIP patch

patches: Re-add `UnwindRegistersSave.S` RIP patch

The previous patch (removed in 7b56f9b17de72d0e237e4af2bf72635c48389f3c)
was erroneous in its implementation but its intent was valid: when
saving registers in unikraft libunwind should take the return address of
the previous stack frame as the IP of the throwing function.
This patch implements this functionality correctly.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Maria Sfiraiala <maria.sfiraiala@gmail.com>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Approved-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #7