[compiler-rt][macOS]: Disable iOS support if iOS SDK is not found

If you are missing the iOS SDK on your macOS (for example you don't have
full Xcode but just CommandLineTools) then CMake currently errors
out without a helpful message. This patch disables iOS support in
compiler-rt if the iOS SDK is not found. This can be overriden by
passing -DCOMPILER_RT_ENABLE_IOS=ON.

Reviewed By: delcypher, thetruestblue

Differential Revision: https://reviews.llvm.org/D133273