Fix that exp(load(x)) is ambiguous

Fix that exp(load(x)) is ambiguous

Because of the template argument added by e34ead15, exp(float) was
preferred over exp(SimdFloat). This causes code which is ambiguous
to compile and creates unintuitive behavior.

Also:
- Add compile tests that these functions don't compile when they
  are used ambiguously.
- Add sqrt to scalar_math which was missing

Change-Id: Ic7582764f1fa9b644d4608536100004e5f737462