CgUtils.fixStgRegStmt respect register width

This addresses a bug found in the armv7a cross compiler. The platforms word with is 32bit, however the code from time:Data.Time.Clock.System

-- | Convert 'UTCTime' to 'SystemTime', matching zero 'SystemTime' to midnight of 'systemEpochDay' UTC.
utcToSystemTime :: UTCTime -> SystemTime
utcToSystemTime (UTCTime day time) = let
    days :: Int64
    days = fromIntegral $ diffDays day systemEpochDay
    timePicoseconds :: Int64
    timePicoseconds = fromIntegral $ diffTimeToPicoseconds time
    timeNanoseconds :: Int64
    timeNanoseconds = timePicoseconds `div` 1000
    timeSeconds :: Int64
    nanoseconds :: Int64
    (timeSeconds, nanoseconds) =
        if timeNanoseconds >= 86400000000000
            then (86399, timeNanoseconds - 86399000000000)
            else timeNanoseconds `divMod` 1000000000
    seconds :: Int64
    seconds = days * 86400 + timeSeconds
    in MkSystemTime seconds $ fromIntegral nanoseconds

resulted in timeNanoseconds - 86399000000000 becoming a native register offset expression (likely during some Cmm folding stage, recognising (MO_Add w reg lit) as reg offset. Subsequently this was then transformed in GHC.StgToCmm.CgUtils.fixStgRegStmt resulting in a 32bit addition onto a 64bit register.