Mask mie/mip fields depending on whether S mode is enabled (misa.S)

[michaeljclark]

When S-mode (misa.S) is not present, writes to mip/mip.SEIP/SSIP/STIP and mie/sie.SEIE/SSIE/STIE need to be masked.

U-mode interrupts and the N-extension are not currently implemented, so these fields are already masked