[AppleObjCRuntime] Don't bother looking for the runtime on non-apple targets

Summary:
This short-circuits the GetObjCVersion function to avoid iterating through target modules on
non-apple targets. This function is called on every Process::IsDynamicValue call, so this
overhead is not negligible.

Reviewers: clayborg, jingham

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D13948

llvm-svn: 251004