Inserted gemv beta-scaling bug into compat layer.

Details:
- BLAS has a peculiar bug (or feature) whereby calling gemv on a vector
  y of non-zero length and a vector x of zero length results in no action.
  Given that the operation is y := beta*y + A*x, many (most?) individuals
  would expect vector y to still be scaled by beta. BLIS, when called
  natively, handles these cases intuitively (with beta scaling).
  Unfortunately, many BLAS test suites actually check for the way this
  situation is handled. Therefore, we have decided to implement this "bug"
  in the compatibility layer so as to provide "bug-for-bug" compatibility
  with BLAS.
Implemented dsdot() and sdsdot() in compat layer.

Details:
- Replaced "not yet implemented" error messages in dsdot() and sdsdot()
  with actual implementations. (These routines are so rarely used that
  this log message will probably lead to some people learning of their
  existence for the first time.)