threads: implement init of TLS and stack pointer (#342)

threads: implement init of TLS and stack pointer (#342)

* threads: implement init of TLS and stack pointer

* fix: rename wasi_snapshot_preview2_thread_spawn to wasi_thread_spawn

Signed-off-by: Harald Hoyer <harald@profian.com>
* fix: change signature of wasi_thread_start

Signed-off-by: Harald Hoyer <harald@profian.com>
* fix: pthread_exit for WASI

Can't use `exit()` because it is too high level.
Have to unlock the thread list.

Signed-off-by: Harald Hoyer <harald@profian.com>
* fix: initialize struct pthread for the main thread

Signed-off-by: Harald Hoyer <harald@profian.com>
* fix: store the aligned stack minus `struct start_args`

Signed-off-by: Harald Hoyer <harald@profian.com>
Signed-off-by: Harald Hoyer <harald@profian.com>
Co-authored-by: Harald Hoyer <harald@profian.com>