NMI is masked by MSTATUS[MIE]

Created by: Imperas

@davideschiavone @MikeOpenHWGroup I think there is a bug with the implementation of the NMI interrupt

The interrupt is only taken if the MSTATUS[MIE] is enabled, this is not correct as NMI is by definition 'non-maskable' From the privilege spec

Non-maskable interrupts (NMIs) are only used for hardware error conditions, and cause an immediate jump to an implementation-defined NMI vector running in M-mode regardless of the state of a hart’s interrupt enable bits.

Thx Lee