[ASan] Add CMake configs for libclang_rt.asan_iossim_dynamic.dylib

CMake changes to build the ASan runtime for the iOS simulator. This is a universal library targeting the same architectures as the OSX ASan runtime does, thus the iossim version can't live in the same universal libclang_rt.asan_osx_dynamic.dylib

The difference between the OSX and iossim builds is in the -mios-simulator-version-min and -ios_simulator_version_min flags that tell Clang to compile and link iossim code.

The iossim runtime can only be built on a machine with both Xcode and the iOS Simulator SDK installed. If xcodebuild -version -sdk iphonesimulator Path returns a nonempty path, it is used when compiling and linking the iossim runtime.