Resurect preprocessor definitions that got lost moving to HostInfo

When building without XCode on sytems where these constants are
not in the system header (or I presume with older versions of XCode),
these are needed to make this file compile, since unlike most other
uses of MachO specific constants, these use the system headers
rather than the LLVM-defined ones.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@216332 91177308-0d34-0410-b5e6-96231b3b80d8