Remove Platform usages from NativeProcessLinux

Summary:
This removes the last usage of the Platform plugin in NPL. It was being
used for determining the architecture of the debugged process. I replace
the call that went through the Platform plugin with a lower level call
on the ObjectFile directly.

Reviewers: tberghammer

Subscribers: uweigand, nitesh.jain, omjavaid, lldb-commits

Differential Revision: http://reviews.llvm.org/D21324

llvm-svn: 272686