[XLA:CPU] Use VectorSupportLibrary for LLVM IR implementation of tanh

No behavioral change intended; this is only refactoring.

VectorSupportLibrary was added after the LLVM IR implementation of tanh so the
tanh implementation was not using VectorSupportLibrary.

The main impetus for this change is that I'm about to add LLVM IR
implementations of Exp and Log, and those are going to use VectorSupportLibrary.
I did not want to have an inconsistency between the tanh and exp, log.

PiperOrigin-RevId: 184774860