[LLDB] Fix TestTargetXMLArch's expected arch

Summary:
When running on an architecture other than x86_64, the
target.ConnectRemote() part of the test may add platform information to
the target triple.

It was observed that this happens at Process::CompleteAttach() method,
after the platform_sp->IsCompatibleArchitecture() check fails.
This method then calls platform_sp->GetPlatformForArchitecture(), that
on a Linux machine ends up returning a generic Linux platform, that then
ends up getting added to the original target architecture.

Reviewers: clayborg, labath, jasonmolenda

Reviewed By: labath

Subscribers: alexandreyy, lbianc

Differential Revision: https://reviews.llvm.org/D44022
Patch by Leandro Lupori <leandro.lupori@gmail.com>.

llvm-svn: 327981