[nnc] Use sleef if its symbols are available (#51187)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51187
Instead of relying on #ifdefs, we want to use sleef if its symbols are
available. This diff adds the mechanism to do that check using LLVM's symbol
lookup.
This diff by itself is a no-op, because sleef isn't properly being exposed to
LLVM yet (the `#ifdef __AVX__` checks are always false, because torch/jit isn't
built with `-mavx`). The next diff will properly expose the symbols, and
perform run time checks.
ghstack-source-id: 120614091
Test Plan: `buck test //caffe2/test/cpp/tensorexpr:`
Reviewed By: Krovatkin
Differential Revision: D26096206
fbshipit-source-id: 3f2b37500276e8bf50a167ecf8aeeb295d7ec232
