Convenience attr constructor for a thread with a small stack

Our sched::thread makes it rather difficult to create threads with
non-default attributes. This patch makes it easier to create a thread
with a non-default stack size, e.g., a light thread with a one-page stack:

   sched::thread a([&] { func(); }, sched::thread::attr(4096))

We should probably overhaul the sched::thread constructors at some
point to make it easier to specify options, but for now, this
specific constructor is convenient for my uses.

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

include/sched.hh

@@ -282,6 +282,7 @@ public:
         cpu *pinned_cpu;
         bool detached = false;
         attr(cpu *pinned_cpu = nullptr) : pinned_cpu(pinned_cpu) { }
+        attr(size_t stacksize) : stack(nullptr, stacksize), pinned_cpu(nullptr) { }
     };
 
 private: