NFC: Refactor library-specific mappings of scalar maths functions to their vector counterparts

This patch factors out mappings of scalar maths functions to their vector
counterparts from TargetLibraryInfo.cpp to a separate VecFuncs.def file. Such
mappings are currently available for Accelerate framework, and SVML library.

This is in support of the follow-up: https://reviews.llvm.org/D59881

Patch by pjeeva01

Differential revision: https://reviews.llvm.org/D60211

llvm-svn: 358001