In TestAsyncPreempt, the function being tested for preemption,
although still asynchronously preemptible, may have only samll
ranges of PCs that are preemtible. In an unlucky run, it may
take quite a while to have a signal that lands on a preemptible
instruction. The test case is kind of an extreme. Relax it to
make it more preemptible.
In the original version, the first closure has more work to do,
and it is not a leaf function, and the second test case is a
frameless leaf function. In the current version, the first one
is also a frameless leaf function (the atomic is intrinsified).
Add some calls to it. It is still not preemptible without async
preemption.