Intercept clone to handle CLONE_CHILD_CLEARTID

[davidchisnall]

The CLONE_CHILD_CLEARTID flag to clone turns the return value into a futex that is signalled on thread exit. We should intercept this and register the futex location with the lthread library so that it can be zeroed and sent a wakeup when the underlying lthread exits. We need to also intercept set_tid_address in the same way.

[davidchisnall]

Once #259 is merged, this should be possible. We need to:

• Add a field on the lthread structure to store the exit futex location.
• On thread_destroy_host, check that field and, if it is non-null, zero the value at that address and wake the futex.
• Add an interceptor for the clone system call that sets this field in the newly created futex.
• Add an interceptor for set_tid_address, which does the same thing.

Carrying the value between the clone interceptor and thread_create_host is non-trivial. The simplest thing to do is to store the value in the stack. The distance between the sp value and sp-128 is the red zone, but anything after that is fair game according to the ABI. We can store the futex address (or 0) at sp-136 and then check this in thread_create_host.

[davidchisnall]

This is no longer required. If we move to using the LKL futex implementation for all of userspace, this works already.