Provide `__set_thread_area()` as glue code

Provide `__set_thread_area()` as glue code

`__set_thread_area()` is a function that is usually
architecture dependent, it sets the tls pointer in the
fs register. An `arch_prctl` system call is used. We
are replacing this call with a custom version because
`arch_prctl()` is not implemented in Unikraft at the
moment.

Signed-off-by: Dragos Iulian Argint <dragosargint21@gmail.com>
Reviewed-by: Florin Postolache <florin.postolache80@gmail.com>
Reviewed-by: Simon Kuenzer <simon@unikraft.io>
Reviewed-by: Cezar Craciunoiu <cezar.craciunoiu@unikraft.io>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Reviewed-by: Robert Kuban <robert.kuban@opensynergy.com>
Approved-by: Simon Kuenzer <simon@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #9