Add OPENBLAS_NO_AVX build option to disable AVX kernels

useful for compiling with OPENBLAS_DYNAMIC_ARCH = 1 using old binutils versions

ref https://groups.google.com/forum/#!topic/julia-users/t9oR1Z_ziM0 and #7240 and https://github.com/JuliaLang/julia/issues/7363#issuecomment-46927356