virtual/{blas,cblas,lapack,lapacke}: add virtual packages.

virtual/{blas,cblas,lapack,lapacke}: add virtual packages.

  These virtual packages are used by the BLAS/LAPACK runtime switching
  mechanism.

Closes: https://github.com/gentoo/gentoo/pull/12323
Closes: https://bugs.gentoo.org/373613
Closes: https://bugs.gentoo.org/381801
Closes: https://bugs.gentoo.org/498490
Closes: https://bugs.gentoo.org/563674
Closes: https://bugs.gentoo.org/564546
Closes: https://bugs.gentoo.org/565776
Closes: https://bugs.gentoo.org/646316
Closes: https://bugs.gentoo.org/657984
Closes: https://bugs.gentoo.org/659014
Closes: https://bugs.gentoo.org/659264
Closes: https://bugs.gentoo.org/669644

Signed-off-by: Mo Zhou <cdluminate@gmail.com>
Signed-off-by: Benda Xu <heroxbd@gentoo.org>