clang/win/gn: Pass -msse flags to clang-cl.exe

This is a port of https://codereview.chromium.org/504893003/ to gn.

(AVX is already correct, see https://codereview.chromium.org/866273003/)

BUG=491209,82385
R=tomfinegan@chromium.org

Review URL: https://codereview.chromium.org/1166693006
1 file changed