xcode: disable avx512 support
xcode: disable avx512 support

xcode reports clang version 9. It does not appear to support inline
assembly using avx512 instructions.

Tested with cmake libyuv on Mac. Fails without this patch.

BUG=libyuv:789

lemenkov/libyuv,lemenkov/libyuv,lemenkov/libyuv,lemenkov/libyuv