Big-endiann support for fastpow

[ChrisRackauckas]

From @YingboMa:

I am not sure about its behavior for big-endian machines, because there are lines like

Float32(reinterpret(Float64, UInt(0x3ff00000 + k) << 32))

[oscardssmith]

Julia doesn't support big-endian machines (largely because they don't really exist anymore).