Missing `NaN128` constant

In the same way that `Base` exports the constants `NaN16`, `NaN32`, and `NaN64`, corresponding to the types `Float16`, `Float32`, and `Float64`, Quadmath.jl should export a `NaN128` constant of type `Float128`.

To match [the definitions in base/float.jl](https://github.com/JuliaLang/julia/blob/master/base/float.jl#L8), we should define `NaN128` as `reinterpret(Float128, 0x7fff_8000_0000_0000_0000_0000_0000_0000)`, i.e., positive quiet NaN with all-zero payload. Note that [we already do this for `Inf128`](https://github.com/JuliaMath/Quadmath.jl/blob/master/src/Quadmath.jl#L403) but not `NaN128` for some reason.