uk_init_tls: Return tcb pointer in __uk_copy_tls

uk_init_tls: Return tcb pointer in __uk_copy_tls

Translate the tlsp to the tcb pointer before using as TCB.

On x86_64 both pointer are identical, but in aarch64 the tlsp
points into the last 16 bytes of the TCB.

Signed-off-by: Robert Kuban <robert.kuban@opensynergy.com>
Reviewed-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Reviewed-by: Razvan Virtan <virtanrazvan@gmail.com>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #6