RFC: Translate Givens rotations from LAPACK to Julia.

As mentioned in https://github.com/JuliaLang/IterativeSolvers.jl/pull/13 it would be good to have stable Givens rotations so here are `xlartg` in a Julia translation. I decided to translate them because of their simplicity and they also seem faster in julia. In addition, they can be used with `BigFloat`. I have not documented them yet but will do so when we agree on the interface.