the TLS), which led to preempt_counter not being intialized to zero and
therefore preemption not being enabled for new threads.

Such non-preemptable threads can monopolize their CPU and cause other
threads with the misfortune of being assigned to this CPU to never run.

This patch adds a simple test reproducing this bug. Because we don't have
an implementation of __tls_get_addr(), we can't "extern" a TLS symbol
(here sched::preempt_counter) from the test, and so I had to create a
new sched::get_preempt_counter() function which I can "extern" from the
test.