fix duplicate extern sdot and missing flags (#41195)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41195
`BLAS_F2C` is set in `THGeneral.h`.
`sdot` redefined with double return type in the case that `BLAS_F2C` is set and `BLAS_USE_CBLAS_DOT` is not.
Test Plan: CircleCI green, ovrsource green
Reviewed By: malfet
Differential Revision: D22460253
fbshipit-source-id: 75f17b3e47da0ed33fcadc2843a57ad616f27fb5
