Initialize pthread structure fields for non-pthread threads

Initialize pthread structure fields for non-pthread threads

If these fields are not initialized as expected by musl, then calls
to pthread functions such `pthread_getspecific` can return unexpected
values or even crash.

Signed-off-by: Marco Schlumpp <marco@unikraft.io>
Reviewed-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Reviewed-by: Sergiu Moga <sergiu.moga@protonmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #41