Fix AVX detection with clang-cl (#35653)
Summary:
Defining macros `/D__F16C__` or sth similar won't work on clang-cl.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35653
Differential Revision: D20735878
Pulled By: ezyang
fbshipit-source-id: 392a664b0a9e74222b1a03b8c3f6ebb2c61d867e
