Interrupt attach patch (2)

[Dave Lawrence]

> This should not happen. Generally, on hardware where interrupt
> enable/disable is implemented by setting the priority level, the HAL
> maintains the priority in RAM and only installs the selected level in
> the hardware when the interrupt is enabled.

I now believe it was all caused by a glitch in the FPGA logic where 
sometimes attempting to disable an interrupt would actually enable it 
instead.    I have managed to write some tests which prove the FPGA can 
malfunction in this way.  In short I think all of the code in 
cyg_interrupt_attach, hal_interrupt_update_level and our platform 
extension is functioning correctly.  I had basically missed the point 
that hal_interrupt_update_level consults cyg_hal_IMASK_table before 
deciding whether to enable or disable the interrupt.

I don't believe this patch to be necessary if everything else is working 
correctly.