SVML support for log2

Although LLVM supports vectorization of loops containing log2, it did not support using SVML implementation of it. Added support so that when clang is invoked with -fveclib=SVML now an appropriate SVML library log2 implementation will be invoked.

Follow up on: https://reviews.llvm.org/D77114

Tests:
Added unit tests to svml-calls.ll, svml-calls-finite.ll. Can be run with llvm-lint.
Created a simple c++ file that tests log2, and used clang+ to build it, and output final assembly.

Reviewed By: wenlei, craig.topper

Differential Revision: https://reviews.llvm.org/D86730