Only build iOS runtime libraries when the iOS SDKs are available.

This was already set up for the iOS simulator ASan dylib simulator,
and this change extends that to the other iOS runtime libraries.
This is in preparation for building all those iOS libraries against
the real SDKs instead of the fake SDKs in compiler-rt.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221614 91177308-0d34-0410-b5e6-96231b3b80d8