Changed postEvent last argument from "nat from" to "StgWord64 other".

Changed postEvent last argument from "nat from" to "StgWord64 other".
StgWord64 other better represents this argument since it can be used as
any particular data, thus "other" and not the "from" capability as it was
previously strictly used.  Also, StgWord64 is normally larger than type
nat to allow for larger data to be passed through the "other" argument.