Initialize TLS and reserve TCB space

Initialize TLS and reserve TCB space

This commit adds a call in Makefile.uk to reserve the space
needed for the LIBC TCB (e.g. `pthread` structure)
and adds 3 glue code files:
`__uk_init_tls.c` - initialize an Unikraft TLS
`__uk_unmapself.c` - allows unmapping for detached threads
`__set_thread_area.c` - bypass for arch_prctl() syscall for
setting the tls pointer

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