Darwin: update default iOS version to 5.0

Defaulting to iOS 3.0 when LLVM has to guess the version is no longer a useful
option and can give surprising results (like tail calls being disabled).

5.0 seems like a reasonable compromise as a platform that's still interesting
to some people.

rdar://problem/15567348

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196912 91177308-0d34-0410-b5e6-96231b3b80d8