Fixup Windows build breaks for the llgs upstream.

Also moves NativeRegisterContextLinux* files into the Linux directory.
These, like NativeProcessLinux, should only be built on Linux or a cross
compiler with proper headers.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@212074 91177308-0d34-0410-b5e6-96231b3b80d8