iOS simulator has the option to build with xcode instead of clang.
GN use_xcode_clang=true enables the xcode build.
As of version Xcode 9.2, the clang version used does not support
AVX512.  The version reported is version 9, but for normal clang,
version 7 is sufficient to AVX512.
When a version of XCode does support AVX512, the version check can
be updated to allow AVX512 for newer versions of XCode.
with XCode 9.2 the following macro is set.
__APPLE_CC__ 6000

Bug: libyuv:789
Test: gn gen out/Release "--args=is_debug=false target_os=\"ios\" ios_enable_code_signing=false target_cpu=\"x86\" use_xcode_clang=true"
Change-Id: I5a9a0b4a2760c7d09a4bcb464b3668979113b07e
Reviewed-on: https://chromium-review.googlesource.com/991595Reviewed-by: richard winterton <rrwinterton@gmail.com>