Fix the issue where the kernel poll code would place the tracking
struct on the caller stack and share it with other threads, thus
creating a cache coherence issue on systems where KERNEL_COHERENCE is
enabled.
This works by eliminating the thread backpointer in struct _poller and
simply placing the (now just two-byte!) struct directly into the
thread struct.
Note that this doesn't attempt to fix the API paradigm that the
natural way to structure a call to k_poll() is to use an array of
k_poll_events on the CALLER's stack. So it's likely that most
"typical" k_poll code is still going to have problems with
KERNEL_COHERENCE. But at least now the kernel internals aren't
fundamentally broken.
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>