When constructing an ArchSpec from a MachO cpu type and subtype, don't set the OS for x86_64 and x86 in case the binary ends up being for macosx or ios.

<rdar://problem/17819272> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@214188 91177308-0d34-0410-b5e6-96231b3b80d8