Circumventing cblas_sgemv segfault in Accelerate

http://article.gmane.org/gmane.comp.python.scientific.devel/19141
https://github.com/numpy/numpy/issues/4007

If dlsym fails the broken sgemv cannot be found because I have overshadowed the symbols sgemv_ and cblas_sgemv. The only way to get the original one is through dlsym. That is also how libVecFort solves a similar issue. I am aborting on error because I would otherwise have to change _dotblas.c to raise an import error. But the idea was to avoid changes to that file.