sched: copy the tls image to a new thread, instead of zeroing it

With the current memset(), every thread starts out with zero-initialized
tls variables.

Switch to memcpy(), so it gets the proper static initializer.

Fixes conf-preempt=0.

arch/x64/arch-switch.hh

@@ -80,7 +80,7 @@ void thread::setup_tcb()
     assert(tls.size);
     // FIXME: respect alignment
     void* p = malloc(sched::tls.size + sizeof(*_tcb));
-    memset(p, 0, sched::tls.size);
+    memcpy(p, sched::tls.start, sched::tls.size);
     _tcb = static_cast<thread_control_block*>(p + tls.size);
     _tcb->self = _tcb;
     _tcb->tls_base = p;