If an executable file is specified with no architecture and it contains more than one architecture select a compatible platform if all architectures match the same platform.

This helps us "do the right thing" when loading a file without having to specify an architecture. 

<rdar://problem/18021558>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@216115 91177308-0d34-0410-b5e6-96231b3b80d8