[irq] Add NMI handler

[tyama501]

Hello @ghaerr

This is the NMI for PC-98 discussed on #2398 (comment)

Although the real NMI is not detected,
I have made int02 test program with C86 on the PC-98 and confirmed the routine is working.

Thank you.

[ghaerr]

Hello @tyama501,

Thank you! Pretty cool how you used C86 on ELKS to test :)

I am currently working on ELKS interrupt code and may make a minor change to this code after commit, in an upcoming PR.