COP0_SYNC
In R5900 mtc0, tlbr, tlbp, tlbwi, tlbwr must be followed by sync.p.
if defined MIPS3_5900, COP0_SYNC is defined as sync.p. else nothing.
IPL_ICU_MASK
mask interrupt directly ICU instead of SR.IM.
I've added this feature to support software interrupt for R5900.
and this option may be useful for platform which has cascaded ICU.