macppc, powerpc: retrigger deferred DEC interrupts from splx(9)

macppc, powerpc: retrigger deferred DEC interrupts from splx(9)

On PowerPC, by design, you cannot mask decrementer (DEC) interrupts
without also masking other interrupts that we want to leave unmasked
at or above IPL_CLOCK.  So, currently, the DEC is left unmasked, even
when we're working at IPL_CLOCK or IPL_HIGH.  If a DEC interrupt
arrives while we're at those priority levels, the current solution is
to postpone any clock interrupt work until the next hardclock(9) or
statclock tick.

This is a problem for a machine-independent clock interrupt subsystem
because the MD code, e.g. decr_intr(), ideally shouldn't need to know
anything about when the next event is scheduled to occur.

The most obvious solution to this problem that I can think of is to
instead postpone clock interrupt work until the next time our priority
level drops below IPL_CLOCK.  This is something we can do from the MD
code without any knowledge of when the next clock interrupt event is
scheduled to occur.

So:

- Add a new boolean, ci_dec_deferred, to the PowerPC cpu_info struct.

- If we reach decr_intr() when the CPU's priority level is too high,
  set ci_dec_deferred, clear the DEC exception, and return.

- If we reach decr_intr() and the CPU's priority level is low enough,
  clear ci_dec_deferred and do any needed clock interrupt work.

- In splx(9) (there are three different versions we need to update),
  check ci_dec_deferred.  If it's set and our priority level is
  dropping below IPL_CLOCK, raise a DEC exception.

Tested by me on PowerMac7,3 (openpic).  Tested by miod@ on PowerMac1,1
(macintr) (`make build` completes).  Tested by gkoehler@ on an unknown
PowerMac (probably openpic).

With lots of help from kettenis@.

ok gkoehler@ miod@