Fix avx for c++14 (#28207)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28207
Enabling c++14 causes these lines to fail with the error "error: the last argument must be an 8-bit immediate".
So let's make them an 8 bit immediate before we enable C++14.
ghstack-source-id: 92419812
Test Plan: Enabling C++14 before this PR shows the error, after this PR does not.
Differential Revision: D17975236
fbshipit-source-id: aa53cdb2d38d89ede2212ed7374fedeb5896f254
