Fix some issue with running the test suite remotely on iOS.

- Added a new "--apple-sdk" flag that can be specified on Darwin only so the correct cross compilers can be auto-selected without having to specify the "--compiler" flag.
- Set SDKROOT if needed



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@222153 91177308-0d34-0410-b5e6-96231b3b80d8