Merge "Changes to public.bzl to support building on iOS with objc_library bazel target." into master-skia

https://skia.googlesource.com/skia/+/d2ccc772d4fb4d1ad97024f95d61de79e4c9954e

Change-Id: I36bd87c7ab180179a419ec8d04c0085df641efaf
FIXME: Failed to generate makefiles!