kernel: Add "meta IRQ" thread priorities

This patch adds a set of priorities at the (numerically) lowest end of
the range which have "meta-irq" behavior.  Runnable threads at these
priorities will always be scheduled before threads at lower
priorities, EVEN IF those threads are otherwise cooperative and/or
have taken a scheduler lock.

Making such a thread runnable in any way thus has the effect of
"interrupting" the current task and running the meta-irq thread
synchronously, like an exception or system call.  The intent is to use
these priorities to implement "interrupt bottom half" or "tasklet"
behavior, allowing driver subsystems to return from interrupt context
but be guaranteed that user code will not be executed (on the current
CPU) until the remaining work is finished.

As this breaks the "promise" of non-preemptibility granted by the
current API for cooperative threads, this tool probably shouldn't be
used from application code.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>