Use the new iOS deployment target command-line options.

We now rely on the -mios-simulator-version-min option to identify the iOS
simulator target. I'm not sure if there's anything in compiler-rt where that
matters, but it's the right thing to do regardless.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@180163 91177308-0d34-0410-b5e6-96231b3b80d8