Sched: fix race start_early_threads()

In issue #145 I reported a crash during boot in start_early_threads().
I wasn't actually able to replicate this bug on master, but it happens
quite frequently (e.g., on virtually every "make check" run) with some
patches of mine that seem unrelated to this bug.

The problem is that start_early_threads() (added in 63216e85978ea2f38)
iterates on the threads in the thread list, and uses
t->remote_thread_local_var() for each thread. This can only work if
the thread has its TLS initialized, but unfortunately in thread's
constructor we first added the new thread to the list, and only later
called setup_tcb() (which allocates and initializes the TLS). If we're
unlucky, start_early_threads() can find a thread on the list which still
doesn't have its TLS allocated, so remote_thread_local_var() will crash.

The simple fix is to switch the order of the construction: First
set up the new thread's TLS, and only then add it to the list of
threads.

Fixes #145.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>