Blas scal cleanup

Clean up of `BLAS.scal!`.  It is a update to https://github.com/JuliaLang/julia/pull/8499.  Issue is discussed in https://github.com/JuliaLang/julia/issues/8415.

1. `BLAS.scal!` is no longer a method for `StridedArray`s

2. Behavior for real-scalar, complex-array is moved to `dense.jl`