Add clang-cl arm64 builds

Nothing too tricky...
   1) tell clang-cl to --target=arm64-windows
   2) work around minor libpng issue temporarily

Change-Id: I4f0d792438610268821b67b92caf08fd78dcec4f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/208882
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
4 files changed