The example wasn't fully implemented the intention - it didn't work as
expected when the trap/proc_exit was executed on the main thread,
because main thread never waited for all the threads to start.
This syscall doesn't need allocating stack or TLS and it's expected from the application
to do that instead. E.g. WASI-libc already does this for `pthread_create`.
Also fix some of the examples to allocate memory for stack and not use stack before
the stack pointer is set to a correct value.
For now this implementation uses thread manager.
Not sure whether thread manager is needed in that case. In the future there'll be likely another syscall added (for pthread_exit) and for that we might need some kind of thread management - with that in mind, we keep thread manager for now and will refactor this later if needed.
