Support the hardware-assisted AST on news68k in the common interrupt
stubs.

Consistently use "intr_depth" as the name of the interrupt depth counter
on m68k platforms.

Increment and decrement idepth in the assembly stubs, not C code. This
provides two advantages:
- Greater coverage for detecting "interrupt time".
- More flexibility for platform-specific interrupt stubs that might need
to do special processing before calling the common dispatcher.

- Declare idepth as volatile.
- Provide a m68k_intrvec_intrhand() routine that returns the interrupt
handle for a given vectored interrupt. XXX This is gross and should
burn to the ground, but is needed to support legacy ISR interfaces.

Add a common m68k interrupt dispatch implementation.