Add library dependencies for thread support

Add library dependencies for thread support

Select (by default) the core libraries used by musl:
`LIBUKMMAP` - provides mmap() syscall
`LIBPOSIX_PROCESS_PIDS` - provides pid,tid implementation
`LIBPOSIX_PROCESS_CLONE` - provides the clone() syscall
`LIBPOSIX_FUTEX` - provides futex() syscall
`LIBUKSCHED_TCB_INIT` - allows setting the size for the LIBC TCB

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